Both good quality ukes, but the choice is definitely the Gretsch, for two reasons:
1. Gretsch has a solid mahogany top. I have personally owned the same uke - it projects really well. The best ukulele in this price range.
The Oscar Schmidt is a laminate wood instrument.
2. Gretsch has a higher quality electronics system (Fishman with built-in tuner), whereas the Oscar Schmidt has a cheaper, generic system (UK-2000).