Schaffenhausen or Stein am Rhein?

So, full disclosure, I've only been to Stein am Rhein, so I can't compare. But I can 100% endorse. Stein am Rhein is gorgeous, and I think it would be awesome to spend a night there, and linger on the beautiful, frescoed town square when it is largely just locals. St. George's Abbey is a wonderment, so allow time for that. You should also have time to ride up and down the Rhine a bit. I think it is a great place.

To a degree it depends what kind of Rhine you want to be nearby.

Stein am Rhein is a lovely town, good high street, close to the river, which at that point is calm and peaceful. I have visited it by boat and train.

Schaffhausen is near the Rhinefalls, and surrounded by Germany on most sides. The Falls is quite impressive, but no Niagara, and the hilly area around very nice. It is more of a city and because of the falls attracts more tourists. I spent a very pleasurable 3 nights at the Schaffhausen youth hostel, one of my favourites in Europe, but that was last century and a bit ago.

Do you have a car? I mention that because there are other things to do, like Aarau and Augusta Raurica. I liked Stein am Rhein, but it's a bit touristy for a full day. I only mean, how many knick-nacks can you buy? I'm not a waterfall expert, but I found the Rhinefall itself a bit underwhelming. Opinion.

I would say one day is right for Schaffhausen and Rhine Falls. It was an afternoon in the city and the next morning to visit the falls and left at noon. Check out some videos to get a feel for the falls. The boat ride was fun to go into the mist of the waterfall. The amount of water flow varies widely. May-June have the best flow. We enjoyed our stay at Hotel 2B,