Check page 2 here - we have an active Marriott vs. Starwood thread going with quite a bit of analysis already. Search for the same thing on the Starwood board - it's a fairly common question.

Marriott points and Starwood points are two very different currencies. There isn't one right answer - either program can be optimal for you depending on your paid stays, award stays, expectations if you hit elite, and credit card usage patterns. (Among other things...)