These top quality pianos are similar in length, 211-212 centimeters long. Marcus compares them in this video.
A fully restored Steinway model B grand piano is in the top price band for a fully restored piano, whereas a handmade Yamaha S6 is rarer.
Both pianos are in immaculate condition, the 2006 Yamaha S6 was hardly used by its previous owner, and is as it was when purchased. The Steinway has undergone extensive restoration including new parts.
Steinway about wins on tone and, and tonal colour. Yamaha wins on touch. However if you are thinking of future investment value, Steinway wins outright. Steinway has a dominant name, fetching a higher value even when 100 years.