I regularly ride some pretty technical trails in the Rockies (not that I'm an expert or anything, but I LOVE it
) Anyways, I've ridden everything (I rented a bunch before I bought a bike) and I prefer my front shock and seat shock over dualies. I agree with spinner that uphill it is harder to get traction, and they do add weight. Actually, no one I ride with has rear shocks, but to each their own.
Have fun picking out your new bike...