Marriott gets a slight edge in terms of the "basic" points earned per dollar for a Plat in both programs. (20 to 5, and I tend to peg them at about 3:1 in terms of value.)

But...Starwood tends to run juicier promos, and the 500pt Plat amenity is (by my own 3:1 ratio) three times better than Marriott. Since I do a lot of one-nighters, those 500 pt amenities add up.

Marriott has a fantastic award - the Travel Package - if you hang in there long-haul to 300,000+ points. Starwood offers a lot of good awards along the way...Cash & Points being just as good as (if not better than) the popular 5-night awards.

Marriott gets the edge if you need a lot of the long-stay or suburban/rural type properties. Starwood gets the edge of you tend to go mostly full-service urban hotels and enjoy suite upgrades. Marriott has an overall higher property count. They have different pockets around the world where one is strong or one is nonexistent.

I find that Starwood adheres to its "no blackouts, any standard room is yours with points" more often than Marriott. My classic example is Oktoberfest in Munich: SPG points = award room. MR and HH points = nada. I've had similar experiences at other major (but not quite the Olympics) events. Starpoints get rooms. Marriott points, for me, have failed. Even the Plat override to pay rack rate for a room at Marriott has failed.

I was once 80-20 Marriott-HHonors. Now I'm probably 70-20-10 Starwood-Marriott-HHonors.