The main pros and cons of the Starwood AmEx, as I see them:
Advantages:
1. 25% bonus miles if you convert 20,000 SPG points at a time, at least to AA.
2. Until you convert, you have Starwood points to use on hotel stays if you want, and with some flexibility as to what they become if you don't.
3. Two-stay, five-night head start on SPG status if that matters to you.
Disadvantage:
1. AmEx isn't accepted everywhere. You need a Visa/MC in addition.
I'd say to go for both, if your credit rating can absorb a small temporary hit for the two accesses in a short time. Might as well get both sign-up bonuses (boni?).