I honestly have to go with Stan. Fed will be 2 weeks shy of 39 when he plays his next major and nobody has come close to winning a slam at that age in the Open era (and no, Sport, Rosewall making a slam final at 40 is not getting close, since he was blown away in that final).
Stan could conceivably win another USO if the field is depleted and he enters Stanimal mode. Fed has virtually no chance to win another USO.