I would go for Platinum. 48-hour reservation guarantee is very useful. Platinum amenity is a nice touch at regular Marriott hotels (though disappointing at the lower-tier Marriott brands in my opinion). I find that special requests for room location, type of room, etc., will generally be honored and front-desk personnel will make an effort to honor requests at check-in if asked. Almost always I will get a nicely upgraded room even on the cheapest room rates (no suites though).
I stay almost exclusively at full-service Marriotts so my experience may be different than someone who stays at the some of the other Marriott brands. The few times I have stayed at Courtyard, Residence Inn, etc., I generally noticed nothing special in room assignments.