Carbon frames are generally lighter and stiffer. You get less feedback from the road, although I don't buy that stuff about carbon's magical shock absorbing ride for a second. In the end, I think there's more in the design and process than the material. You can't say a carbon frame will always be better than an alloy one or vice versa. I've ridden some good carbon frames (Cervelo, Madone, old S-Works) and some bad ones (Planet X) and can say the same about alloy frames. As usual with bikes, you get what you pay for. Find a decent shop and have a ride before you buy.