I heard Key West is generally a bit nicer and has a better pool. We chose KW over Bella this fall when we upgraded a unit. Both are mandatory so it's important to figure out if you are getting either. NOTHING else in Florida is mandatory. The seller should advertise it accurately and tell you which it is in. They can get this info easily by requesting an estoppel from Starwood, which they should have before they sell the unit anyway.
As Denise said, SVV (Bella, Key West, St Augustine...) is different than the million SVR resorts that are also on eBay so be careful what you buy. SVV's mandatory section usually goes for $5000 ish whereas SVR units can go for $1.
Katherine