Well, these places are WAY out of my price range, so I can't comment on that part.
But I've spent a LOT of time visiting Italy. So I CAN talk about location.
Rome: St. Regis is actually a convenient location, around the corner from Bernini's Ecstasy of St. Teresa, the bulk of the archeological museums, etc. Termini is close enough to the hotel to be convenient, far enough that you don't feel like you're in the sleazy terminal area. Fendi would be in a quieter neighborhood, but not as close to any major attractions. However, it might be more central overall, since it's closer to the Tiber and Vatican Museums.
Florence: the St. Regis's location sucks, IMHO. You have walk a long way to get to the sights, and the Piazza Ognissanti isn't that scenic. The only reason to go way over there is to see the Church of the Ognissanti, and its most famous work is only viewable very limited hours. The Portrait looks much better located, near the Ponte Vecchio where all the tourists are--and the reason they're all there is because it's near the center of everything.
Venice: interesting case. The Aman is near the Rialto Bridge, and I kind of hate that area. It's always such a mob scene. But of course, Venice itself is a mob scene. It is, however, pretty much in the center of everything. There's a vaporetto stop not too far away, so that's good. The Gritti is close to San Marco in a well-traveled touristy area (more touristy even than Rialto, but doesn't bother me as much, maybe because it's more open and less claustrophobic). But the Gritti is such an iconic hotel, I'd certainly want to stay there just to say that I had.
So there's my 2 cents.