I'd think income tax is also a strong factor.
On Bill Simmons' most recent podcast, he asked Daryl Morey what is most important. His answer: The first, second, and third most important factors are the players that a free agent would be playing with. Everything else is secondary.
I will trust that Morey knows more than me on this subject. I also agree with him. The most talented players want to win championships. They can vacation anywhere they want, and the marginal tax difference over a couple years pales in comparison to winning or losing.
I'm not talking about the secondary players, but the top 25 or so. They want to win.
Fortunately, Boston is set up to win with a little bit more roster improvement.