OK, according to the RCCA mag i was reading last nite (an old back issue) The 2.5 (yes i said the mag was old) Vantage carbon chassis is lighter by something like 60 grams.
Would i use one ? no.
Why wouldn't i use one ?, carbon is ok and all, but alloy is more appropriate for a basher and a rig that is going to take abuse. As was said, leave the carbon chassis for use on a racing rig only as that is where it pays the most reward, other than it is easier to damage a carbon chassis under bashing conditions than an alloy one.
Ask revo_rob_1984, he twisted his alloy chassis, if that was carbon, he may have broken it (though don't quote me on it).