Hi there, right, usually when companies use S,M and L to distinguish their geometries, what that usually mean is the frame is the same off the ground, but differs in length, in terms of the top tube. I'm quite short, unfortunately, but I've got long arms in proportion to my body, so I would ride the M size frame. Therefore, i Could still throw the bike around whilst still feeling in control of it.
So if you interested in the 51cm, go for the small. It will be nimble, you will be able to 'chuck' it around better, plus, because it a slightly smaller size frame, it will lighter, which is what we all want!
I hope this helped.
If you have any further questions, please feel free to ask.
Robski