OKW is more secluded. Both SSR & OKW have walkways to Downtown Disney (as well as boat access), but SSR is much closer. The most secluded section of SSR (excluding the treehouses) is closer to DTD than any part of OKW. At the very least, staying at SSR affords you closer proximity to more dining options; asking to stay in the Congress Park section would you you about a 5 minute walk to the Marketplace side of DTD, and still pretty close to the main house and themed pool (with a quiet pool right outside your door, too). This also makes it faster and easier to resort-hop or go eat at restaurants in resorts, since you can walk to the Marketplace bus stop and then hop a bus to any other resort.
One advantage OKW has (I'm pretty sure) is that they have watercraft rental on site. I've not seen watercraft rental at SSR, and according to allears.net, you can acquire about watercraft rental at SSR's "Horsing Around Rentals" where you can rent stuff like surrey bikes. Which means they probably get you to OKW or Riverside so you can rent a boat. Don't know if that's even a thing you want to do, but there you go...
SSR has the spa on-site. In case either or both of you want any sort of treatment, it's right there...
OKW is more secluded. Since it's just you two lovebirds, that may be advantageous.
Choose wisely.