I've owed a number of both Ruger and Smith's. This is only my opinion, but the only thing a Smith has over a Ruger is perhaps a better trigger (well, they're a bit easier on the eyes as well ).
The Redhawk is simply a stronger gun - period. There's a reason why loading manuals have "Ruger Only Loads" and not "Smith Only Loads". I don't buy into the whole "you'll shoot a Smith loose after so-many rounds" thing, but I do have more confidence in the fact that my Rugers are just plain tough.
A 4" barrel will be a bit handier when packing out in the woods and the fps you lose compared to a 6" won't really matter to whatever you come up against.
Both companies have excellant CS, so if anything goes wrong, they'll square you away.
I'm just a bit biased towards Ruger's as I shoot some pretty stout loads and I know that it can handle them. I know the Smith could as well, but perhaps for not as long.