Frankfurt or Zurich

Frankfurt is a much bigger airport, but if flights are all the same to you, Zurich is a much more interesting city.

Frankfurt was flattened by the War and rebuilt with modern skyscrapers. Its nickname is Mainhatten (after the Main river it sits on). There is just one small square left called the Altstadt that has anything close to charm.

Zurich is an entire city of beautiful wonders. Very walkable. Make sure you take a cruise on the lake.

I would select Zürich because there are, IMO, superior options for your 3 or 4 days. In fact, you could stay in Luzern and still go right to the airport easily the day you fly out as long as your flight isn’t at 0-dark-thirty. Luzern makes a good base for the area and even facilities a day trip to the greater Berner Overland region. It is early for the mountain activities like hiking (April) but you can still go up to great vistas and it is lovely in the valleys. I urge you to read up on Luzern and what you might enjoy doing there, mountaintops, city sights, etc.

I do not understand what shall be interesting in Zurich?

Rhein-Main area around Frankfurt allows imo much more selection of activities and offerings for various interests.

I trust you know that Switzerland is very expensive.

https://community.ricksteves.com/travel-forum/switzerland/interesting-town-near-swiss-german-border

OTOH, train daytrips from Frankfurt are very easy. But it is hard to do 2 in a day unless they are on the same train line.

If they're finishing a Rhine River cruise, then they will have already seen the beautiful areas of the Rhine. I vote for not backtracking, and flying home from Zurich. Actually, we stayed in Luzern after the cruise because we wanted to spend time in the mountains, not in a cosmopolitan urban environment like Zurich or Frankfurt. The Zurich airport is only 1.25 hours by train from Luzern.

Where are you stopping on your cruise? That will help.

Please stop telling people that Frankfurt was flattened in the war. It most certainly was not. Only a very small part was bombed in the inner city. All of the neighborhoods that were built turn of the century are still there as are all of the stone churches that were NOT destroyed, nor was the gorgeous Alte Oper.
If you want historic, Frankfurt has tons of it waiting for you. Medieval churches (and older, dating back to 850), cloisters, Jewish heritage, lively Farmers markets, a neighborhood that is on the Half-Timbered Route in Germany, location for the election of the Holy Roman Empire as well as many coronations, site of the 1st freely elected parliament, archeology from the Celts and Romans that lived here and dozens of museums.

I'm sure for residents it is still very pleasant. However:

Frankfurt was severely bombed in World War II (1939–1945). About 5,500
residents were killed during the raids, and the once-famous medieval
city center, by that time one of the largest in Germany, was almost
completely destroyed. It became a ground battlefield on 26 March 1945,
when the Allied advance into Germany was forced to take the city in
contested urban combat that included a river assault. The 5th Infantry
Division and the 6th Armored Division of the United States Army
captured Frankfurt after several days of intense fighting, and it was
declared largely secure on 29 March 1945.[26] Frankfurt consists to
over 40% of buildings from before World War II, besides all
destruction.

Since you end up in Basel, which is a good town to explore on it's own, I recommend you fly out of Zurich. But don't plan to spend the 3-4 days there, instead take the train over to Lake Konstanz and spend them there. You have Mainau, Lindau, Meersburg, and all the attractions around the lake to keep you entertained, and you cna't see them all in the time you'll have. You can take the tram in Dornbirn up into the Alps and watch the sunset while you have dinner. You can take a ride in a Zeppelin. You can see the extensive paleontology exhibit in Bodman. There's a lot of stuff to do. The food is good. It's cheaper than Zurich. And it's totally different than the Rhine.

I don't know Frankfurt, but am far more inclined to believe a city resident and guide, like Ms Jo, than a partial uplift from a Wikipedia entry. My already low faith in Wikipedia dipped again last week when I saw that one of the big money clique of editors had written on his profile that he "detests original research because it goes against all that Wikipedia is about".
That says everything to me about Wikipedia. All research in old books or wherever was original once upon a time and facts don't stand still. As a local historian such statements don't sit well with me.

Even reading the whole Wikipedia article, not a partial extract with bad grammar into the bargain, and seeing the pictures in it, gives a contrary impression.
That is without looking at a city tourism website which doubtless exists.

darrenblois, I'm sure for residents it is still very pleasant. However:

The however leads to what looks like an uncredited quote as it seems to have a (nonreferred) footnote.

From whom were you quoting?

Much of London (and other towns and cities in England and Scotland) has been rebuilt after enemy bombing - do you tar them the same way?

40% of Rothenburg is rebuilt or reconstructed, but it doesn't stop people from going there, nor Rick from claiming it is the best preserved town in Germany (it isn't)

If you have to put a roof on a building or install new windows, it wasn't destroyed. That is what they did with the Alte Oper and the Kaiserdom.

Have you not wondered why only 5500 people were killed with such extensive bombing, when cities like Cologne, Dresden, or Hamburg lost 30,000-50,000 people. Because they mainly used phosphor bombs which are deadly on wooden buildings (the alt stadt) but not on stone buildings. It was a bombing meant to demoralize the population.

There was very little fighting in Frankfurt in March 1945. The Americans were coming down the Rhein from Worms and Oppenheim, the Nazis blew up all the bridges in Frankfurt, so the Army Corp of Engineers built a piece of bridge that they rolled down the street onto the bridge and the troops walked across. (I have the pages from a diary that describe this) The guy in charge of Frankfurt was killed on his 1st day of duty by an American bomb (the previous guy had left the day before) so there was no one to push the Nazi soldiers to fight. The population was tired of the war and were happy to not have to get up in the middle of the night for a bombing raid. The Americans walked in on the 28th and the war was over on the 29th.

So, that was too much information for a thread like this, but I just get really tired of people believing the whole city was destroyed and that there aren't any beautiful old buildings here.

Both cities have great sites nearby. Neither cities are top drawer for touring in my opinion. If you had Munich, Vienna or Berlin, go with one of those.

Also, Switzerland is more expensive than Germany.

Its a matter of opinion. If you all think Frankfurt is a pleasant medieval town, go ahead, enjoy it. I went there, enjoyed the Romer, but the rest of the city is indistinguishable from Detroit. It is glass skyscrapers, concrete plazas, and non-descript railway lines. I don't happen to like it because it is modern. I didn't make up the Mainhattan nickname. Its a common perception, shared by millions. That doesn't mean it can't be for you. People from Detroit love Detroit, and should. That doesn't make it a charming tourist town.

As for the comparison to London, it is night and day. If all of London were like Canary Wharf, it would be a dreary place indeed. London was far less destroyed by the War than Frankfurt was. The problem with Frankfurt is it is ALL Canary Wharf - at least the parts within easy tourist access of the hauptbahnhof (except, as noted, the Romer district).

An objective look at Frankfurt 1945 - and the Frankfurt which evolved afterward - can be obtained by browsing through the many dozens of aerial (and other) photographs at the Columbia University site at the link below... if you are interested.

https://www.columbia.edu/~fdc/.tmp/ffmaerial/frankfurt1944.html

I never said it was medieval, only that there are some churches and cloister that are.
If it has been years (like 30 years) since you have been here, then you might be surprised at the changes.
If all you saw were skyscrapers, you didn't see much. The neighborhoods of Bornheim, Nordend, Westend, Höchst, Sechbach, Fechenheim, etc. are lovely with almost exclusively turn of the century architecture as well as their fair share of half-timbered houses.

So, am leaving this discussion. All I wanted to do is point out the pluses of Frankfurt to those who don't really know anything about it. Hopefully, someone who knows Zurich well can do the same for that city.