Which would you choose, and why?
I think it depends on what you want. I like both very much, and while I happen to have a slight favorite, that's based on my own preferences.
Resort "vibe": OKW seems to whisper "island time" to me every time I turn around, with very lush landscaping. SSR has a sense of understated elegance, and has finally grown into its landscaping after early years when the buildings seemed out of scale relative to the treeline.
Both are peaceful respites from the parks, though OKW is a little more so by virtue of not being next to Disney Springs. The golf course winds through the buildings at OKW more than in SSR, which may or may not be a good thing. I'm not sure if it's really true, but SSR
feels a little more densely populated than OKW does; that may be due to the building design.
Dining options: At OKW, Olivia's is one of my favorite restaurants anywhere in WDW, though I suspect some of that is nostalgia. On the other hand, Goods is probably my least favorite counter/window service location. Anything else requires a drive, bus, or (when it is running) boat. At SSR, I still haven't managed to eat at Turf Club, and I miss having a table service restaurant for breakfast/lunch. But Artists' is one of the better counter service spots, and Disney Springs has tons of options in SSR's backyard.
The pools at OKW--even the feature pool--feel like an afterthought, and the walking paths are almost all along roads, which makes them a lot less pleasant. SSR has two very good feature pools, and for the most part the walking paths are separated from the roads by the buildings.
I tend to stay in the villas rather than studios, so this difference might matter less, but I
really like the OKW 1BR layout. The rooms are more spacious, feel more comfortable, and OKW is one of the few resorts at which the patios/balconies are big enough to really relax. However, if I were looking at studios, the bed + sofabed at SSR is more functional than OKW's two beds for a couple.
Finally: if you'd asked me four or so years ago, I would say OKW without hesitation. But, two of my last three visits were to SSR, and I've found it has really grown on me. Since you haven't tried SSR yet, why not give it a shot?