Heidelberg or speyer

It depends on your interests? Do you want to see where one of the original Jewish communities in Germany lived (includes an ancient deep, mikvah), plus a gorgeous ancient cathedral, or do you want to see a castle, learn about Mark Twain and his visit here and perhaps a student prison?
(no idea what a tour of each city is being offered by your ship)

Perhaps tell us what the tour descriptions are. I like Heidelberg a lot, but I also find Speyer very interesting.

Two cities I really like, for completely different reasons. I visit both regularly.

Heidelberg is bigger, much more touristy, and always bustling. I love it for the art galleries, bookstores (it's a university town), and shopping. On a clear day you have great views up and down the river. The walk along the river is excellent in the mornings or around sunset. the public transit is extensive and easy to use. I don't know why more tourists don't visit the zoo. If you've never been there don't get off the train at the main station, take it all the way to the Altstadt stop and then walk into town along the river. Heidelberg has a very good, spread out, Christmas market.

Speyer is totally different. I never use the public trans. I drive, because I know I'm going to stop along the way. The Technical Museum is a must for anyone who like cars, planes, trains, boats, spacecraft, and huge mechanical music machines. I go twice a year. Try to attend Brezelfest, when they actually drives some of the collection through the streets. The cathedral is the burial place of more Holy Roman Emperors than anywhere else. The State museum right across the parking lot is good. The old town is easy to walk along, and has a lot of interesting places to stick a head into. (I spent a couple hours helping hand set type for a couple vintage printing presses one day just because I could.) Why Speyer can have a nice beer garden on the river and Heidelberg doesn't I can't say. They also have a very interesting (small) museum on the Celts, who lived in the area before the Romans.

Both of these cities are far nicer than Worms, which a lot of tours on the river stop at.