Two other things to consider are where you would like to redeem your points, and who has the better credit card:

Marriott definitely has more properties in the U.S. including many very nice resorts. As mentioned above, the Ritz Carlton redemtion option is nice although costly points-wise.

On the other hand, Starwood has much more luxurious resorts and good locations worldwide. Their 75 luxury properties come close to the quality of a Ritz anyway.

As far as credit cards go, Starwood's Amex is one of the finest available and clearly beats out the Marriot Visa (although the free Cat. 4 night is nice).

Hope this helps,
J