I own at both resorts. My current favourite is Marbella Beach Resort for the following reasons: No longer under construction, slightly larger villas, better location although it's easier to access Playa as not so much car weaving to do around the resort's entrance. Elviria (home to MMBR) is really nice and you can walk places. Generally more interesting with shops etc.
Playa is closer to Puerto Banus (not that Marbella is particularly far) and in an area that is currently less established than MMBR. Playa has a fantastic iondoor pool and the gym overlooks it. Playa is much more Andalucian in style ie terracotta floor tiles vs marble. Beautifully decorated. Indoor changing facilities superb. Has its own beach but no-where to go for a leisurely stroll. MMBR is a public beach where you can enjoy some nice walks.
MMBR has a restaurant. Playa is getting one. MMBR has a tapas bar, not sure about Playa.
You won't be disappointed with either resort. Playa is currently cheaper to rent from Marriott as under construction.