Richter had both Steinways, and Bösendorfer pianos, and later in life, he played only on Yamaha grand pianos, and was sponsored by Yamaha, Japan.
Now, let us have a look at this intriguing question. To begin with, let me report an anecdote here. There are both classical and Jazz pianists who swear on Bösendorfer instead of Steinway. Let me mention only the Austrian pianist and Mozart specialist Paul Badura-Skoda and the late Oscar Peterson. (Bösendorfer actually produces two versions of tuning and intonation for their piano, one for classical, especially Mozart, Beethoven and Schubert, and one for Jazz—the latter version is often played with the grand completely open).
Now the case is especially interesting regarding Svjatoslav Richter who was sponsored by Yamaha in his later years. He had no particular liking for Steinway and the anecdote goes that when recording Schubert’s Wanderer Fantasie back in 1963, he was playing it in on a Steinway first. Then, late in the afternoon, when the studio stuff was about to go home, he changed his mind to record the entire piece again on a Bösendorfer—and asked them to stay until late at night.
He played it in again, that same day, on a Bösendorfer and that was the final version, which he approved and which is one of the most striking proofs of his musical genius and pianistic abilities. It’s a truly monumental take on this extremely difficult piece. I guess that what inspired him to achieve so high was not only his inner mind but also the fantastic piano. You can listen to it here (including score):
Richter Plays Schubert Wanderer Fantasie on Bösendorfer
Paul Badura Skoda about Bösendorfer
But I would like to mention in addition here a piano maker from Italy, FAZIOLI, as well as one from Korea that the American pianist Earl Wild preferred, the KAWAI CONCERT GRAND.