I have the small swivel and wouldn't climb w/o it. I love the thing. I climb on a master saddle with a bridge i have attached to the outer leg loops, and the swivel is attached to my bridge.
Not sure if you have seen them both together, the large and small. It's just that the large is very large. It is designed to hold more than one carabiner, and for that matter more than one person. I personally think it is over kill, unless your planning on useing a second tie in point, and tieing it into the swivel. Then the large is what you need.
My only complaint is that sometimes it will side-load my carabiner as it slips through the swivel as Iam climbing. All that means is you have to pay alittle more attention to it!!! But my overall rating is about a 9.5 out of 10.