I went skinny on Troppo's advise, and find it heaps better.
As with alloy or carbon, I do freeride slalom with a 6.6 sail in rough ocean, and find the carbon a lot stiffer when overpowered or hit with gusts. I've been breaking alloy booms within a year, for years, as do a lot of guys. But it wont be long, and I'll break even on cost, the carbon has lasted double the time so far and feels better.
As with cycles, a mast is flexing non stop, and a good full carbon skinny will last for donks. A alloy boom is like a spoon, bend it back and forwards enough and it'll break.
Just my thoughts 
Smaller diameter booms are excellent - even for big people!
As for Carbon/Alloy - I have snapped 1 boom in the last 2 years and bent 1 boom in the last year. I did both of them on Narabeen Lake - 1 in a huge Southerly and the other in huge Westerly. Funnily enough they both broke from catapults after hitting sand banks! Thats with Alloy booms. In the waves i've snapped plenty of outhauls (I seem to be a set magnet in the lulls 
) but never damamged a boom. I have seen a few Carbon booms broken in the waves.
If I could afford the best Carbon boom (and had the cash not worry if it snapped) then I'd buy one without any hesitation. However I can't!! I buy booms for about $130 each - it's a big difference.
I have also noticed that a lot of the pro's in the magazines are now using alloy booms.
