Starwood card does not participate in Membership Rewards. $1 charged = 1 Starpoint; 1 Starpoint = 1 mile in a couple dozen airline programs, but Starwood adds a 5,000 miles bonus when you transfer a block of 20,000 = 25,000 miles. However, Starwood does not offer double points for supermarket, pharmacy, gasoline, cellphone or postage. This means you may end up with fewer miles, even with the 5,000 mile bonuses, depending on your spending pattern and which airlines you wish to use.
I suggest you read this thread for a thorough comparison of the two programs...
http://flyertalk.com/forum/showthread.php?t=523503
...in my estimation it isn't a choice, they are both valuable, but Platinum is not the best card for earning MR points.
dennis