On a river cruise and have a choice of Heidelberg or speyer. Which town do people find more interesting
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.