Hi eventemp,
I'm currently the owner of a Baldwin L, but before that had a Steinway M. I pretty much grew up playing Steinways and Baldwins and enjoy them both, so have remained partial to them.
If you are looking at the Yamaha S6 in your possibilities, that piano is 6' 11", of course. So the Steinway B is nearly identical in length. The Bosendorfer 214 is 7' as well as you know. Either the Steinway or Bosendorfer will ultimately have the edge for greater residual values than the Yamaha due to superior materials and craftsmanship in my opinion. (Yamaha dealers will vociferously dispute this though.) I've played Yamaha numerous times. To me the bass is woody and the treble is brittle and bright. Also, the sustain (tone decay) is faster than American pianos, making it harder to effect legato playing. So I'm not too keen on the sound. I do like the Yamaha action a lot though--very even and responsive.
As to which would win the contest between Steinway and Bosendorfer, that's a good question. Larry Fine has found in his latest survey that one difference maker between the two is quality, and Bosendorfer has been winning there lately. In fairness though, as long as the Steinway receives proper dealer prep, that could equalize that particular difference.
One thing I dislike about Borsendorfer is that because the scale design favors the fundamental tones, it is a purer but thinner sound than the fuller and more complex "American sound" which emphasizes the richer overtones in scale design. One result is that when playing at dynamic fff, the Steinway carries it off with thunderous aplomb. The voice of the Bosendorfer seems to croak, crack and shatter like broken glass at those moments. Others here have noticed that too. One person mentioned that when trying a Borsendorfer in a showroom, it was even more startling, in that the voicing seemed altogether different for the bass, alto and treble to the point where he thought he was playing three different pianos!
A suggestion I would have would also be to try a Baldwin SF10E (7') in your explorations. It's a marvelous piano, but not for everybody. Baldwin is a sturdy, artist quality piano that also holds value well. Like Steinway, it requires a lot of dealer prep prior to delivery to the customer. Also, if you come across a Mason & Hamlin BB (7'), sit down and try it too. Many passionately make the case for M&H over Steinway. My first teacher had an M&H Model A, but I liked her Steinway B better. So it comes down to individual taste and preference.
Estate planning is always a smart idea, I agree. But in your lifetime, the primary consideration is that you thoroughly enjoy your piano, especially given the large outlay that you will make to purchase it. So I say put that at the top of your priorities!! Visit dealers and try as many artist quality pianos you can to find the one whose sound you love. Let that be the chief deciding factor.
Good luck in your search!