If you are staying in a 2 bedroom the size difference is most noticeable in the living room, kitchen and laundry room. These are areas you will use alot on your trip then I would recommend OKW.
We have stayed at OKW many times and truely like it. We have stayed at SSR last summer when it opened and loved it. The difference in spaces is in areas we don't need the extra space. The bedrooms and baths we found to be comparable in size. We loved the decor and furniture at SSR so luxurious.
As to the parking situation. That is a toss up too. If you have a unit at the front of one of the buildings at Congress Park and you are lucky enough to get a front row parking space you are really close. At OKW if you are parking area is crowded you might have to park further away from your actual unit than the distance at SSR. So I truely do not think it is fair to say the parking at SSR is further than OKW. It could truely end up being either way and you will not know until you check in.
SSR has a fabulous main pool and OKW has the new slide, which I have not yet seen so I can't compare. The quiet pool at Congress park is the best of any of the quiet pools at
DVC in my opinion. It is large, beautiful and has a great view of Downtown Disney.
Some have commented on Artist's Palette, but this is the one of the main reasons we love SSR. We were thrilled to have a quick place to eat that offered something other than pizza, hamburgers and chicken tenders. We loved the food, the staff was so helpful, making adjustments to the menu as we wanted, and I love the sitting area, so light and comfortable.
You can not go wrong with either, they are just very different.
SSR is still a work in progress and OKW is a finished product. Truely that is the major difference.