I've ridden both, and I'll go for a hardtail every time. I've had a Trek 6000 for 8 years and other than standard things wearing out, it's been great. I've rode it so much I actually wore out the gears!
(Right now it's sitting in the garage disused since an off-road triathlon in September. I bent the rear deraileur hanger. I was going to get it fixed this week, but the BOV on my 951 broke. Priorities and all.
)