I don't think that you can go wrong with either one. Both are large, well-maintained resorts with terrific amenities. OLCC has better mini-golf and an outstanding waterpark feature but, as otherwise noted in this thread, some of these resort features are far apart and may be inconveniently located depending on where your villa is situated. However, there are pools located in each phase of the resort. At Vistana the amenities and villa phases are more condensed and the pool areas are pretty convenient to each phase, though maybe not as impressive as those at OLCC. There's a resort convenience store at both of these resorts but there's a Target store across the road from OLCC and a Publix store that's at the edge of the OLCC property right at the east entrance to the resort.
So, perhaps, it would be a matter of location. OLCC is located in Kissimmee near the Animal Kingdom Lodge and the AK theme park, the closest park - it's about 5 miles from OLCC to the AK parking lot. The Sheraton Vistana Resort is located on SR 535 on the other side of I-4 from the Downtown Disney area. It's about 5 miles from Vistana Resort to the closest theme park, Hollywood Studios. Vistana is a little bit closer to more Disney theme parks than OLCC is but not by a lot. But OLCC somehow seems further away, at least for me.
For us the Vistana location is better because of the proximity to DTD, the Premium Outlets, several of the area restaurants that we like and it's close to two entrances/exits to I-4.
Dick Taylor