Both resorts are pretty wheelchair friendly, and have ADA Compliant rooms available. If you are getting a studio, SSR will have more open floor space simply because SSR has a queen bed and full sized sleeper sofa, OKW studios have 2 queen beds.
Also, only 3 buildings at OKW have elevators, so you'd likely be on the first floor at OKW. This is because ADA access laws changed between the time most of OKW opened, and construction of the final 3 buildings (62, 63 & 64).
Overall, I'm sure you'll be happy with either resort. Like all Disney resorts, the busses will have lifts or ramps. There is a boat from OKW to Downtown Disney, and it accomodates wheelchairs, unless the water level is way too low to load a motorized chair safely. There are also busses to DtD.
SSR also has a boat to DtD that also stops at the Treehouse Villas.
If you need an ADA Compliant room (roll-in shower, raised restroom facility, etc) be sure it is booked though Disney's Special Needs department when you make your reservation either through CRO or DVC.