First off, all the studio villas at WDW have a queen-size bed, plus a pull-out couch that turns into a queen-sized bed. So they all have two sleeping surfaces.
Though I have walked around the grounds, I have never personally stayed at Old Key West. The atmosphere there is nice, but it doesn't "wow" me. I do have to say that the OKW fans are a loyal bunch. Former Moms Panelist Laura swears by the size of the OKW villas and has to beg to get her family to stay anywhere else. The main plus I hear about OKW is that the for the money, the rooms are the largest at WDW.
I have stayed at Saratoga Springs. The resort is much newer that OWK, so everything is quite fresh there. My room at SS was well-appointed and very quiet. On the plus side, SS is convenient to Downtown Disney and the spa facilities there just can't be beat. On the negative side, the place is HUGE. I was at WDW last week with my hubby. We ended up driving around SS a bit because we were investigating the Treehouse Villas and visiting the DVC sales office. We got lost at SS several times because it is just so darn big. You can read more about my SS experience
HERE.
I am wondering why these are your choices. If you are a DVC member, I would choose the studio villas at the BC, BW, Wilderness Lodge, Animal Kingdom Lodge, or Bay Lake Tower (even without a view) before I'd choose either SS or OKW. Each of those choices has either a better location or better hotel amenities. If you are not a DVC member, unless you reeeeally need the microwave, I would probably stay in a regular room in either a moderate or deluxe resort over either SS or OKW, again for the better location and amenities. Just something to consider.