Medieval Russia or Ancient China?

The Middle Ages spanned about a thousand years. Most historians would classify the "Chinese Medieval Period" as the span of time between the beginning of the Three Kingdoms Period to the rise of the Ming Dynasty, which would also work out to being about 1,150 years. Think about the vast differences between life in 2021 A.D. and 1021 A.D. Now think about how vast both China and Russia are. Without knowing specifically when you intend to set your story as well as knowing where in particular you intend to set it, no one can answer that question. A great deal happened over the course of a thousand years in both places. Do you know enough about either to even begin to write something that might come off as being even remotely plausible?  

Both have made excellent settings for a fantasy. "Inspiration" is a personal matter. If YOU are not "inspired", it doesn't mean someone else might not be.
If you have a story to tell, tell it. If you haven't, nobody here cares whether the story would have been set in Russia or China, as you're not going to tell it anyway.

That would really depend on one's interest.

There are fantasy novels out there in both settings that have been well-received: From the top of my head, Katherine Arden's "The Bear and the Nightingale" is an example for the medieval Russia setting; Guy Gavriel Kay's "Under Heaven" an example for ancient China.

I was going to write that China is more exotic but quite frankly most folks (me included) probably know almost nothing about medieval Russia, either, so that might be just as exotic but in a different way.

There will be readers who are more interested in one setting over the other, some who would like both or don't place too much importance on the setting, and some who wouldn't care for either.

Choose whatever fits the story you want to tell.

It doesn't matter. What matters is what you do with it.

What's wrong with Polynesia,  the Mayans or ancient Cambodia (their waterway trade routes were extensive and lined heavily with cities in the 1200's)

Well, either would be good, but why not go with something more exotic than that? Both have been used in fantasy settings before, and Leigh Bardugo's Grishaverse with its semi-Russian setting is very visible today with the TV series being made based on the books.