I'm not sure whether there is a "right" answer here. Here's what I'd take into consideration:
- how flickable you want the bike to be
- how your height is distributed
- your style of riding
The smaller bike is going to be more flickable and feel a bit more nimble, IMHO.
Is your height mostly in your legs, or do you have a longer torso? Longer torso would typically indicate that you'd go with the larger size.
The type of riding you are going to do is going to make a huge difference. That ended up being the deciding factor for me more than anything else. If you tend towards more technical riding, go with the smaller frame. Less technical, go with the bigger frame.
As a point of reference, I am 5'6" with short legs. I ride a custom fit road bike - 50cm seat tube, 54.5cm top tube, 73 degree seat angle. The fit and my form on the bike is perfect.
In contrast, on my small Mojo, I'm riding technical slow singletrack (New England rocks and roots). I started with a 100mm stem and have shifted to a 70mm stem, and added about 1cm of height to the stem during the switch. After the change, the bike feels much more maneuverable and has transformed completely. I've been running 685mm bars the whole time. Weight balance is such that a lot of times, if I dig my front wheel into an obstacle and stop, the rear of the bike will rotate, but I won't crash (happened just this weekend, in fact...) I never would have been able to get it dialed in properly with a medium sized frame.
However, for cross country, less technical riding, my setup sucks and I'd be better off with a medium. Your mileage may vary.
Albert