A marathon bike should have less travel and be lighter than an all mountain bike.

all mountain is just that..all mountain, you could do some XC on it and you could do some light downhill/freeriding on it. its not ideal for either but an ok mix.

XC marathon iis for xc type stuff, single track, fire roads, general fast xc riding.

dont get an all mtn bike unless you think you are going to be hucking it. an xc mountain bike is a much better choice for a triathlete.

you wouldnt want to do xterra or an endurance race on an all mtn bike.

All-Mtn will be too heavy and suck at climbing.

All Mountain bikes are a step down (on the huck-o-meter) from downhill bikes. All of those bikes have 6" or more of travel. That's a LOT of suspension, and it, undoubtedly, weighs a lot. Full suspension XC race bikes are generally in the 3" - 4" of travel range, with 3" still being very common. I would expect a "marathon" bike, as GT has it broken down, to be in the 4-5" range.

If you're going to race at all, don't get an all mountain bike. (Unless you're racing Super-D or Downhill!)

IMHO, just look at the full suspension XC bikes. They are going to be lighter, and most people don't notice the difference between 3" of travel and 4".

just ge a steel hardtail 29er!!!!

Amen...can I get a hallelujah!

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What if the Hokey Pokey is what it is all about?

The all mountain bikes have 6 inches of travel versus 4 inches for the Trail/XC class. Plus geometry, the Force gets a 2 degree slacker head angle than the Marathon. The Marathon will have a lighter build kit. This means the Force will be harder work to keep that front end down going up due to more rear sag, a higher front end and slack head angle, but on the way down its more stable and speed and over obstacles.
It all depends on the riding style you do, and the terrain you ride. You could race an all-mountain bike XC, but I wouldnt want to do it and be competitive. Plus on non technical trails its not so much fun and waay to much bike. On the right trails like National Trail here in Phoenix a good AM bike is a total hoot. You can take lines and drops you wouldnt dare on a XC bike.

Kevin

well yeah, an all mountain bike would be sick at south MTN, except on the desert classic trail.

So, for Xterra races I would want a Marathon series over an all mountain? I had no idea there were so many types of "MTB's"

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"injustice anywhere is a threat to justice everywhere" MLK

absolutely, get a solid XC rig, I know GT makes one. so does specializrd,felt, jamis, trek so you have TONS of choices, dont get an all MTN rig unless 75% of your riding is downhill.

yeah, there are tons of different MTBS.

FWIW, check out a 29er while you are looking, I went 29, I would never go back to 26.

Mate, there are many, many kinds of mountain bike, every nice is filled. All with different suspension travel and geometry. You have XC Race/Trail/All Mountain/Freeride/Downhill in ascending order of travel. Geometry and suspension characteristics are also important. For example a bike could have 150mm of travel, but its geometry may place it more in the Trail camp (I actually think the GT Force is more of a long legged trail bike with its 69 degree head angle) whereas a bike with the same travel but a slacker geometry and burlier build will fall more towards the technical XC/light freeride side of the equation.
For XTerra you absolutely would want to go with a XC full suspension bike, the GT Marathon would be a good choice, it is more of a XC bike than stated trail with its 71 degree head angle. As Ironwolf suggested try a 29'er hardtail. I prefer the 29'er hardtail to the 26" variety, but overall prefer the 26" FS as I race longer events and have back problems and although a 9'er is a bit more comfortable than its 26'er counterpart, its not suspension. If your trails are smoother however you may not need rear suspension. 29'er full suspension is in its infancy, but the new Superfly 100 is a damn impressive bike, and the only 9'er I have ridden that could tempt me away from my Titus Racer X.

Kevin