We have stayed at both, and while I think the SSR main pool area is nice, I think it gets very crowded, and I'd probably prefer to have kids that age at OKW where they can enjoy both the sandcastls slide AND the sand play area. Besides that, each neighborhood pool at OKW has play gyms too.
It really comes down to what unit size you are staying in. If you are in a 2 bedroom, it really doesn't matter all that much, but if you are in a studio, then OKW gives you two real beds instead of 1 bed and a pull out sofa. If you are in a 1 bedroom, it's nice to have the extra space at OKW as well.