The bike store didn't have a small 29er in stock, so I had to try a Medium 29er in the parking lot. It felt great too. But after 2-3 hours of talking with the owner and taking measurements of my body I decided on going with the small. He and I both agreed that I would fit okay on either size bike, but the small was more suited for me and type of riding I planned on doing. Still it's totally up to you.
I'm 5'7" and went with a medium Stumpy HT 29er since a small felt cramped. Only thing I've done is gone to a shorter stem from 90mm to 75mm since I felt my seat was too far forward making the front heavy at times.
'14 Specialized Epic Expert World Cup
'15 Niner RLT9 105
'15 Cervelo R2, Enve 3.4 w/CK R45