Its worth remembering how difficult it is to get a greencard to live in the states, but easy to do so if you have a job there already and how good life is in the states for uber wealthy people(ok its good everywhere, just way better weather in the states).
If I was either of them I'd take a job in the states, as a gateway to being able to live their full time.
Also remember for the person who said walking about in Dubai would be easier than in a competitive USA.
Train in 45c temps and walk around for 90 minutes, then run around for 90mins in 30c heat, MLS will be way easier, and its a more competitive and growing league but frankly those two are still way above the average level in the MLS and will be for years even slowing down. Scholes is still way way way above average in this league with zero pace to speak of.
AS for being more competitive, more skilled players in the MLS are LESS likely to break your leg than an unskilled tired from playing for 80 mins in 45C temps player in the Dubai league, but the simple fact is, 99% of your time in either country will be spent living, not playing, I'd prefer to live in a freer and nicer climate USA than the sweltering, nice for a couple weeks, Dubai without a whole heck of a lot to do.