"Heck, I am 5'6 and 1/2" and just ordered a Small. Sure hope I didnt go too small!"
It just depends on what you're comfortable with. I think 5"6"-ish is one of those could-go-with-either-size heights.
I wasn't able to demo a bike and wasn't quite sure of which way to go. I chose the medium mostly because the bikes I've had in the past didn't really have top tubes much shorter than 23". I figured I could still go medium with a 50mm stem and I'd end up just a bit shorter than previous cockpits which I thought would feel fine.
If I went with the small, I'd probably end up putting a 70mm stem on and things would be alright. I tried a 70mm on this one and it felt a bit too much, 60mm might be okay for longer trail days, but the 50mm seems to be doing fine.
I did get to ride a friend's Syren, briefly, that had numbers similar to the small, and while it was fun on the short downhilly section I got to ride it on, I didn't think I would be able to get used to that for everyday pedaling and long rides. But that's just me and my comfort level.
If you are use to, and like, a bike with more quick and flickable characteristics (or you want to go in that direction) the smaller frame would do that over a larger frame.
Again, you're the size that could go either way, so it kinda comes down to personal preference at that point... at least with the small frame you're gaining a bit of standover. I almost chose the small just for that reason alone.
crazyhouse