Some states base an athlete's taxable income on the # of games, not the # of days. And some states have a special tax just for athletes.
The state tax in Utah and Mass is virtually the same, so there's no big difference unless he's considering the Heat (except all the extra games in CA that a WC player pays probably adds up). If he's in Florida, he could probably expect to pay an avg. state tax of probably about 1/4 to 1/2 of what he's paid before. So if he averaged 5% before, and say it'll be 2% now, over a 4 yr contract, that's a difference of maybe $3 million.
Not insignificant, but you wouldn't think it would be a huge factor in his decision.