if you're new to the sport, i would probably just stick with the 26" for now, as 29"s are pretty pricey. But, if you're into the sport already, and willing to make an INVESTMENT, then go for it. personally, i don't find that 29's are all that great... if it ain't broke, why fix it?
it's not that it's broke, it's just different. 29ers roll better because of the bigger wheel, so they're good for XC, but they're weaker because of the bigger wheel as well, so they're not as good for jumping off of ****.
Hardtail? If you're > 5'10", don't even think about it, go straight for the 29er since you'll just end up ditching the 26er if you buy one first (about 90% probability, anyway, judging from experience).
5'6 to 5'10", try a test ride
5'6 or less, 26er
hardtail 29er plus full squish 26er makes a certain amount of sense
My vote in hardtail 29ers is for the Niner SIR9, EMD9 or AIR9. All fantastic bikes.