San Antonio no doubt.
Better climate, advantageous tax situation, stable organization, and probably the best coach in the game.
I do think it partly depends on the player. There are players out there who might prefer the atmosphere in Boston, both with the history, and the pace of the city, over a place like San Antonio.
The other thing about San Antonio is that even though they have a stable organization and a great coach, they are also kind of like the Patriots of the NBA. They basically play moneyball. They are not player friendly. Doc is a much more player friendly coach.