Depends on the course and the distance. But I think full suspension might generally be the way to go. There is a really great article in triathlete magazine (July 2005) that interviewed Ned Overend, a Xterra (off-road) triathlete and mountain biking pro.
He said full suspension bikes keep your tires on the ground and reduce the bouncing when you hit a bump thus keeping foward momentum. On a hardtail you bounce up or even backward when you hit a bump. It takes more energy and has an impact on your body.
As far as traction is concerned (climbing, braking, cornering), the full suspension wheels are going to follow the ground better. You won't drift through turns,you climb rough ground better and you can brake later.
Ned says that over the last 10 years, suspension bikes have been getting lighter and have more efficient pedaling systems. There are definitely courses smooth enough that a hardtail is all you need. And of course, a hartail will be lighter. But the efficiency of the full-suspension bikes over a wider variety of courses makes it a better choice if you're in the market for one...
To sum it up, the full-suspension bikes are lighter and pedaling systems more efficient, making them more applicable for a wider range of courses.
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I race XC on a hardtail anywhere from 10-40 mile races and my body certainly takes a toll. I'm going to invest in a full-suspension bike for next year. Though I will still keep my hardtail waiting in the wings - just in case.