lots of good advice coming from all kinds of backgrounds...
IMHO - instead of debating FS versus HT (or 29 vs 26), i think its more of a do you want an all-mtn style bike or a more x/c oriented bike. sounds like you are looking more all mtn - something you could beat on if you are on the right trail for it. i think an FS is a great choice... and to throw another idea in the mix - there are all mtn HT's too - like the on one 456 (burly steel frame, one issue tho - welded derailleur (spelling?) hanger), the jamis komodo, or the evil sovereign. cheaper to maintain, and cheaper up front cash layout.
i ride a 29er HT mainly. I could've bought a great FS for what i paid, but i didn't for two reasons. one - FS bikes are more expensive to maintain (in general). there are more parts to break. i blew out my rear shock on my FS ride once - it weren't cheap to fix'd it. two - i feel quicker on a HT on climbs. i like to stand and mash on most climbs, and even on the best FS platforms you still lose something when climbing out of the saddle, and i need all the help i can get going up, so its HT for me. my next ride might be a FS or HT, but i know it will be more all-mtn oriented vs x/c oriented - i do like the on one 456 (it is burly-ass HT). i take my HT 29er to the sourlands and the like and have a great time, but i would like a ride i can just beat the piss out of, versus having to avoid some lines due to bike choice - for example, there's no way i am dropping off anything on my niner - that'll just be a good way to need new parts (like a fork, wheels, or even worse - frame).
if you are buying used check out the frame with a fine tooth comb. look for any signs of cracks. look at the components - do they look totally beat or do they look somewhat maintained? buying used makes sense (i have bought used myself and have had good luck), but just do your best to see any signs of abuse on the bike versus just normal wear and tear. if you are buying a full suspension - check your rear linkages for play too - could be a sign you might need new bearings/bushings - which wouldn't necessarily be cheap.