Definitely go small unless you have real long legs. The top tubes on the SJ 29ers is pretty long and so the bikes feel on the big side. I'm 5'10 and the medium is perfect for me.
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.
Wow, according to the specs, the small has a higher standover height at 750mm vs 741mm on the medium. So, now your inseam height is important. If it is less than 770mm I would go with a medium..
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.