Allez is a great bike. I'd agree that a 1lb difference is insignificant for anyone not racing a bike up a mountain. I would assume the main differences here would be in terms of ride quality. The Allez is, by all accounts, a very nice riding bike that doesn't have some of the sterotypical harshness and buzz that other aluminum frames are associated with.
I have a similar comparison with two bikes I own, but they are both CX bikes. I have an older aluminum Cannondale CAADX and a newer carbon Cannondale SuperX. The biggest difference I notice between them is that the carbon bike is more dampened and feels more comfortable on rough stuff.