Bacharach or St. Goar?

Bacharach even though it is a bit more touristy.

For a quick stop, Bacharach is more scenic.

If you have more time, I really like the Rheinfels castle ruins at St. Goar. The skeletal ruins are even more impressive to me than many standing castles.

It depends on what you want to see the most. St. Goar has the massive fortress Burg Rheinfels as well as a beautiful, ancient church. Bacharach has lots of half-timbered buildings, as well as ancient church.

If it was me, I would go with St. Goar. Simply more interesting. The view from the fortress over the Rhein is stunning.

Ms. Jo really touched on the main differences between the two. Her suggestion is spot on.

taking a river cruise on the Rhine ... and will only have time for one
stop.

Sounds like you are taking a long river cruise. Perhaps you should take the train to Bacharach and spend a little time in Bacharach, then take the cruise down to St. Goar (the most interesting stretch of the Rhein), see Rheinfels castle ruin, then go on by train from St. Goar.

I think visiting the towns is more worthwhile than seeing more of the same kinds of things on the river.

The section of the Rhein between Bacharach and St. Goar has the Lorelei and the Pfalzgrafenstein, two of the most unique features of the Rhein, in addition to some riverside towns and hilltop castles. Anything else, you can see very well from the train.

"I'll be taking a river cruise on the Rhine..."

Which segment of the river do you plan to cruise? I generally recommend Bingen (where the great scenery begins) to St. Goar, a cruise of 1.5 hours.

Map of this segment: http://www.bingen-ruedesheimer.com/images/map/rhine-valley290.jpg

IMO a Bacharach - St. Goar cruise - only 40 minutes long - means you'll see only half the best part; you'll want to see the sights below by boat, I think, which would mean starting in Bingen:

River scene near Bingen: https://upload.wikimedia.org/wikipedia/commons/e/e3/Maeuseturm_Burg_Ehrenfels_Bingen_Rhein.jpg

Rheinstein Castle, just north of Bingen: http://www.welterbe-atlas.de/uploads/pics/burg-rheinstein-trechtingshausen.JPG

View across the Rhine from St. Goar to St. Goarshausen and Burg Katz Castle (the boat is not the cruise boat, but a ferry boat there to shuttle folks across the river.) St. Goar IMO has a more scenic location than Bacharach - you can also see Rheinfels Castle and Burg Maus Castle (further downstream) from the riverfront in St. Goar.

So if you end your cruise in St. Goar as suggested, you'll be "home" - no need to take a train back to Bacharach or elsewhere.

How to see Bacharach AND put this all together... It really depends where you're coming from. Are you landing at FRA just prior? If so, I'd try to make the cruise your first exposure to this area (taking the train first along the Rhine might "spoil" the cruise experience a bit.)

• train to Bingen (about 1 hour or less) - walk to KD dock w/bags, cruise to St. Goar, drop bags at hotel.
• see Rheinfels Castle
• train (10 minutes) to Bacharach, return in evening by train to St. Goar (trains run hourly or better in each direction.)

It might be possible to break your cruise in Bacharach if the boat schedule allows - however, you will have bags to drag with you around town, or you'll need to deposit them with the Tourist Info office, as there are no station lockers in Bacharach.

If you are in good physical shape, climb the stairs to Burg Stahleck in Bacharach, now a youth hostel. They have a terrace with a wonderful view of the Rhein river. From the hostel cantine you can get wine, beer and something to nibble on. To stay overnight, you have to take out a membership see: H.I. (Hostelling International). The rates are very reasonable and breakfast is included. You get to meet travellers from all over the world. The site is very popular - preregistration is a must. Here is Rick Steves' youtube video:

https://www.youtube.com/watch?v=G9VGDU_yIDo

If you only cruise from Bacharach to St. Goar, you will miss seeing all the beautiful castles. This is much too short of a cruise. You have spent a lot of money to get to Germany, spend an hour seeing the stunning scenery. This is not the place to skimp on time.