I've got two Defy models, a Defy 1 and Defy Advanced 2. I bought the Alu first (no surprise there) and then bought the carbon one a year and a half later in the sale for almost the same price. They are both great bikes IMHO.
The Advanced feels a bit more sprightly when you give it some, and maybe it soaks up the bumps a little bit better. Its a bit lighter too, so it makes climbing a bit easier.
The Defy 1 will take mudguards so it's now a very good winter or wet weather bike.
Is there a night/day difference? I don't think so really, they are both excellent bikes. Should I have bought the carbon one first? Probably, but I didn't have the money at the time so my decision was based on that. You won't go wrong with either bike though.