Personally, I would avoid a small of back (SOB) holster. I have considered them myself at various times, but after doing some research and looking for reccomendations, have found no real reason take the leap.
Main reasons I have seen to avoid SOB holsters are:
1) Distance away from potential target- you have officially placed your weapon as far as possible away from any target you face.
2) Difficult/Impossible to draw quickly without covering people in a 90-130 degree arc from your target.
3) Risk of injury if you fall on your back, increasingly risk of back/spine injury.
4) Often uncomfortable to carry, especially when sitting.