Carbon is stronger to an extent. i.e if u took the extact smae designed bars, 1 carbon, 1 alloy, and did a 40ft drop(or something similar) then the carbon ones would just snap clean, where as the alloy ones would bend.....then snap, or bend too much. But since you wont be doing 40ft plus drops or SuperMan sized drop gaps...then go for carbon.
Carbon doesnt fatigue. So after so many years of use, they will remain the same strength as the day you bought them. Unless you take a BIG chunk out of them, then they start to crack, but even then, they last for another month or so. The alloy bars would fatigue, and eventually bend, crack or snap off.
The also have 7075(?)Alumium tubes on the rise of them, and on the end of them. This helps to protect the carbon from chipping at the end, and it stops the carbon(where it is weakest) to snap. The aluminium just makes sure the Carbon fibre stays in tack, and uber strong.
Size wise...if you want to spend £50 on 30'' bars then go ahead, but i think it'll feel horrible on a mod. I've just got the 28'' uns, but Pazzaz.
I think TartyBikes may have some slightly used Pazzaz Carbon Risers 28'' in, for £20? E-mail them <_<