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.