Is it really a question of what game will be better? Seriously?
I have seen many of the clips released on God of War 3 AND Dante's Inferno and the winner is CLEARLY God of War 3.
GOW 3 is first off made by Sony Santa Monica. They're a great developer who's past work has received critical acclaim. Secondly, they pioneered the God of War playing style and, from the E3 demo, they seem to have improved or polished many parts of the gameplay. Thirdly, they have a higher budget and get the advantage of working on one platform.
Dante's Inferno on the other hand, looks to be very much a 'rip-off' of God of War. It also has the disadvantage of not having a memorable character. Dante looks pretty... well, I would say overall Meh... Take a look for yourself.

The environment certainly plays an important role in just about every game. But again, God of War 3 is going to trump here as well. The enviornment is alive. Titans move in the background and destroy shit in a wonderful setting of greek myth. Dante's Inferon looks to have nothing but bland setting of brown going to another bland setting of brown. Nothing is alive, it's all very boring. Or at least that's what I saw from the demo they released.