This is excellent advice. The Amex SPG card is great in terms of redemption for stays at Starwood hotels and for exchanging your SPG points for miles on various airlines. Be aware that the exchange rate for United is worse than for most airlines, though. For UA, it takes two SPG points to get one mile.

You can garner some quick bonus points from SPG by finding links to sign-up bonuses of 10,000 points for both personal and business cards. Plus I think there may be an offer that gets you 15,000 (?) additional points if you spend $15,000 within the first six months, which seems like it would be no problem for you.

I don't have the links handy, but you can find it by searching the Starwood forum here at FT. A good key word would be 10,000.

I also agree with the earlier advice that you probably shouldn't switch airline affiliations based on just one itinerary.