They are both quality pianos, but I think the key is to have them voiced and regulated by a top piano technician. I have recently been to several showrooms of both brands, and was appalled at the lack of preparation of all the instruments. They were uneven in tone, and the actions were very unresponsive in some cases. On one there was even a key that stuck-and it was a new piano! It appeared that all of the instruments were just taken off the truck and tuned-no more than that.
I inherited a Steinway B from my late piano teacher, and it wasn't especially gratifying to play until I had it voiced and regulated by a master technician. This took a couple of days and it cost $800-900 at the time, but it was well worth it. Now it is one of my favorite instruments I have ever played. But I had no idea of what I had until I had this done. It is a shame that piano stores (at least the ones I have been to in southern California) don't invest in this process so that their pianos can be shown to their advantage.
Perhaps if you find a piano you're interested in, you could insist that the dealer prepare it by one of their staff technicians so that you can evaluate it better. Also you could (and should) have a technician you trust look at a prospective piano before you buy.
Again, I think they are both quality brands, and even within each brand the instruments vary a little. It is a matter of personal taste. I happen to like to Steinway better in general, but I have played several Mason & Hamlins that I really liked as well.