Greysword is right, the Amex offer is the best around right now--the Amex points convert 1 to 1 to Delta Skypesos.
But, Skypesos are not good for much, very hard to use for flights--although folks have some luck with Korean to the Orient and Virgin to Oz.
And, occasionally, Amex/Delta have 50-60K bonus offers for the card. I snagged both a personal and business card a year or two ago when that offer came up. Still haven't been able to use the miles.
AA and UAL are much, much more useful.
As to SPG Starwood, Amex often has a late summer offer for 30k bonus. Also keep in mind that you can often get a 5K bonus for a friend by having them refer you if they are SPG members.