If the two of you will be staying in a studio and sharing a bed, book SSR. SSR studios have one queen bed and a sofabed. OKW rooms may have more square footage, but you'll have more space to move around at SSR with the sofabed closed all of the time.
If the two of you want separate beds and will be staying in a studio, book OKW. OKW studios have two queen beds and no sofa.
If the two of you will be sharing a One Bedroom villa, I really don't think it matters. OKW 1Bs are larger, but with just two adults in the unit I can't imagine the extra room would be of much value. I guess I'd give the edge to SSR again for the health club.
BTW, if you're planning to visit for 9 days THIS December, your odds of getting ANYTHING lie somewhere between slim and none. I've been reading posts here for months saying that the DVC resorts are already fully booked. December is a very busy time, probably for the same reasons you hope to visit (low points, Christmas decorations, etc.) and you need to plan more than 3 months ahead to get a room.