I would recommend carbon post only if it is constructed with an alum sleeve. Some alum cores are thicker than others. If you bike came with a carbon post, then don't worry and just ride it.
If you do any kind of jumping,impact related riding, or you know that you are "hard" on your equipment, then I would recommend full on alum posts. But then, you can go carbon, but expect to replace it at least once. It's hard not to take a jump every so often on a mtn. bike.
Also, if you do any kind of long distance riding (40miles) out in the middle of no where, go alum because you don't want to hike back with a broken post.
I've seen carbon posts break at mtn bike short track races, and CX races. Not a big deal, but it goes to show, carbon can break unexpectedly.
Having said that, I do ride a carbon post on my CX and road bike, but alum on my XC dualie. I just recently convinced myself to install carbon bars after months of contemplating over it.
I think with MTBing a carbon post is subjected to a lot of different stresses than road riding. Most of these is due to bouncing around on the saddle. I wouldn't want to take the chance of bouncing on a seat over a rough section and have a carbon seatpost break.
Now I have a carbon on my road bike, but I use an aluminum on my FS bike. Carbon is good if your looking for a weight weenie bike, but with the technology out now, aluminum components are just as light and as strong.
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