Thinking of a river cruise, either in China or Japan. Now I realize these are different but any advice for someone who has never been to that part of the world

I've never heard of river cruises in Japan. Where do they go?
Cruises aside, the Japanese culture and Chinese culture couldn't be more different. Spend some time with a guide book or on the web looking at their art, temples, handicrafts, food, etc and see which appeals to you.
China is huge, so traveling to different regions can be quite time (and $) consuming. Japan has a lot going on in a much more condensed space.
I am less happy when being "hustled" these days. It's rare in Japan to get that feeling. Tips are not expected, prices are usually fixed, and street crime is very rare. Traveling is easy there, and I find it quite relaxing.
China is a fascinating country with amazing history (old and new) but it is a bit more work to get what you want and where you want to go.
Of course these are just random generalizations, so to me, it would boil down to which culture stirs my interest.

You could find a cruise on the Inland Sea of Japan. That and/or around the islands. Rivers in Japan are short.

IMO, a cruise is not the best way to see either country. I agree, you need to spend some time with guidebooks to get a sense of either place.Go to your library and browse. See what catches your fancy.

A "river cruise" in China usally means the Yangtse. I did it back before the water level really went up with the Three Gorges dam, so have no opinion whether it is worthwhile now. However, there is so much more to China than that. There is actually a Smithsonian cruise round Japan plus one stop in Korea friends of mine are taking next year that doesn't look too bad, but I still think you are better off on land. And you really can't compare the countries, they are so very different.

To follow on the idea of the above post, the Yagtze river cruise is not worth it and certainly not a good way to see China. Quite different sonce they raised the water level and, in any case, it is only three days

I wouldn't suggest a cruise on the Yangtze too. Better to visit China on land.

We did the Yangtze before the dam was built and the villages flooded. I can’t begin to imagine all that was lost when the floods were released.
It was our least favorite part of our first trip to China as we felt we had lost the freedom to explore although the villages/towns were interesting.