I think one thing that hasn't been fully discussed in this thread is where you plan to use your points. Marriott has lots of locations as does Hilton. Internationally Marriots properties are usually closer to business centers (like in Rome)
In general I find the lounges to be superior at Hiltons; especially in the USA. (There are exceptions, obviously)
Before you make your choice, I'd take a look at the globe, think about where you'd like to go; and see who has properties where. (like Venice; Marriott has only a courtyard by the airport - though new, its not convienent; Hilton has the Stucky and its awesome. (Though I do like hte Westin Europa and Regina too))..
Much of my travel patters is a reflection of where my next trip is in a year and the promotion offered by the program. Right now I've gotten my first set of 25,000 points for SPG, and about 1/2 way done with Marriotts Mega Bonus. My diamond status is good though March 2010; so I'm not staying at any hilton properties right now (and they have no promo right now either)