As you can see by the above info, Old Key West is larger. But your question was...which is best?
While it's true that OKW is larger, some find having to carry a personal GPS navigation device and packing a lunch to go from the master bedroom to the kitchen a little inconvenient. And then, there's the whole wildlife thing - alligators, bats, tree frogs, deer, racoons, chupacabras, etc.
Seriously, though...all of the two bedrooms are pretty spacious, and all include full kitchens and washer/dryers. Most of the resorts have the stacked washer/dryer combos, OKW has full-sized separate washers and dryers. I don't think there is any such animal as a "bad"
DVC two-bedroom.
If you want max space and a quiet, tranquil setting (except for the darn chirping birds!), OKW is your place.
But you may decide that space alone is not your only criteria. If you want to be in the middle of the Epcot/MGM/Boardwalk action, you'll find very nice 2-bedrooms at BCV and BWV. Want to be close to Downtown Disney? SSR is beautiful. Close to MK? VWL.
Your choice -- all good choices.