Can not make up my mind.

Oh, wow. I've done both (very similar) trips, and both were wonderful. Tough decision.

My second trip to Europe included Budapest, Vienna, Prague, Czesky Krumlov, Salzburg, and Munich. (Age 56)

My third trip to Europe included Lisbon (with day trips to Sintra and Cascais), Madrid (with a day trip to Toledo), Barcelona (with a day trip to Montserrat), and Nice (with day trips to Monaco and Cannes). (Age 57)

Lisbon is one of my favourite cities ever. Ever. Loved it so much. The food was amazing. The people were wonderful and welcoming. And it was less expensive than some other places in Europe. Sintra was so fun and interesting. I also loved Madrid and Nice. I did not love Barcelona, but I'm glad I saw some of the wonders there, especially the Gaudi stuff. On the other hand, I found Montserrat to be one of the most breathtaking, magical places I've ever seen.

Prague was number one on my "bucket list." I enjoyed it a lot, and it is exquisitely beautiful. I adored where we stayed (The Questenberk). However, I was surprised by how much I liked Budapest. I didn't know that much about it. I added it to my itinerary, because I was using river cruise itineraries as the basis for my own planning (but adding twice as much time in each place and spending half as much). But Budapest was amazing and fascinating! (Mister E's tips were sooo helpful in making a visit to that city easy and enjoyable.) The architecture was stunning, and I learned a lot about the varied history of this crossroads to so many important events and eras. I also really liked every place else on the trip, except for Salzburg. (That was probably not Salzburg's fault, though. I didn't spend enough time there to see much, and I wasted half of it trying to find lost train tickets.) I was in Amsterdam for a few days in 2021 and loved it. (Consider staying in a bridge house, if that would be suitable for your situation.) (Age 63)

So, I'd say that you can't go wrong either way. However, if it were me, I'd probably choose the more Northern route. I don't like heat, and I'm guessing it would still be quite hot in Lisbon and Madrid at that time of year. It was 40C when we were in Madrid in May of 2015, although we visited a lot of galleries and museums, which had air conditioning, so that helped.

That said, as I stated above, you really can't go wrong with either itinerary.

I agree with BB. As senior travelers, choosing a trip can be fun. In this case, you have the choice of two fantastic itineraries.

When we have a dilemma like this, we flip a coin; or we use our “travel jar,” which has ideas on slips of paper inside it. We shake the jar and pull out a piece of paper. Whatever is on the paper is our next trip. And we are always grateful for a next trip at this stage of life.

I'm with the flip a coin suggestion. Both are great itineraries, so that's a good a reason as any if there is no obvious attraction to either for you.
How early in September? That may help determine if heat will be a factor or not.
How about cuisine? Is that important to you?

Have you looked at Rick Steve's videos on each of these places, available on this website?

You can easily spend 2-3 weeks in Portugal. A good source for more information is Julie Dawn Fox blog. I love Portugal, the people are warm and welcoming.

https://juliedawnfox.com/

Could also take an easy flight to the island of Maderia from either Lisbon or Porto to add some variety. Beautiful.

I have been to all these places and I think the Amsterdam, Vienna etc itinerary is better for that trip. Spain and Portugal each deserve 2 weeks to see beyond the big cities.

I would also pick up north because of the heat in the south. All of these cities are worth a visit.

I vote for the 2nd option mainly because it includes Vienna.

Vienna is always a joy to go back for repeat visits. Just about each trip since 2009, I've managed to stick Vienna somewhere in the itinerary. I did that this 2nd time on a three week trip by making a detour but still staying for 3 full days after arriving on the night train before 7 AM.

Vienna is intriguing, linguistically interesting, culturally and historically significant and captivating pertaining to the Ring and the outlying districts, where you see only locals, regardless of their ethnic origins, very easy to get around by using a combination of the trams, S-Bahn and U-Bahn subway systems, cuisine is diverse, if you're looking for that, or just staying with traditional Austrian fare.

They really are totally different trips.

Why don't you build iteniaries for both, pick accommodations, activities, transportation, etc. Then you will really know what you are choosing and the price, and you have one plan for now, one future.

I personally preferred Spain and Portugal to your other itinerary( except I love Amsterdam!) I am glad we visited Prague,Budapest and Vienna but have no interest in returning to those cities. Food has something to do with that as I find Central European food quite heavy, not a favorite!

I would watch as many videos as possible and decide. See what the emphasis is in the majority and then consider the season you are traveling. Here is a start: https://youtu.be/1nd5AtZIrTk?si=pFRCsEvhi0njaMbt

OMG, I have trouble spending less that a week in most cities. Last October we just did a week in Madrid with side trips to Merida and Segovia over one week. This October we did Cordoba, Granada, Ronda, and Sevilla over 2 weeks. Next year we are looking at doing 2 weeks covering Roman ruins from Rome to Paestum (but if fear it is too much).

I would suggest getting a few sheets of paper and a few guide books (they are cheap compared to airfare), developing a high level itinerary for each potential trip and then decide based on what you think you would find most enjoyable. Then you can always use the other itineraries later.

Since someone mentioned food. One of the great adventures of travel is eating the local food. No denying that. But sometimes what gets overlooked when visiting major cities is the availability of great food of every variety. Not all of us come from a U.S. big city so having exposure to Georgian or good Greek food might not be possible. So, if its Paris or London or Berlin or ..... if you are going to be in a major European city, do what the locals do and eat Indian or ..... ?? I put together this list for Budapest, but any number of great cities in Europe can open you up to the same options: https://community.ricksteves.com/travel-forum/hungary/restaurant-alternatives