Good luck with Starwood on this one. I've been a Plat with both for a while now, and for the most part Marriott lounges are better than those at Starwood hotels.

Also note that while most every non-resort FS Marriott has a lounge, Starwood FS hotels quite often don't have one at all (i.e. I do believe only Sheraton has a lounge as a brand standard.)