I have both with matching Rossi Model 92s. I'm very fond of both and would be hard pressed to pick one over the other. For hunting I prefere my RBH in 45 Colt. Loaded hot I feel it is a more devestating round but for SD purposes I'd lean towards the .357. The GP 100 being double action and the ammo being much lighter and easier to carry.
Both will do the job. I guess I'm not being much help here as I have no plans to pick just one.