Zinn says:
I have done numerous crank length tests for VeloNews and the results show that (1)there is no one crank length that works for a given body size and (2) standard stock crank lengths are arbritrary. It's worth experimenting to see what you like.
Personally as you're only 5ft 11 I reckon 172.5mm would see you right, but you do have quite a long inside leg for your height (I think), if you intend to do a lot of hill climbs then maybe track down a 175mm crank length they do exist for 105 (I have one on my road single speed).
5700 is the newer version of 105 which looks (and I guess has some of the design elements) a bit like Ultegra/Dura-ace…not sure why it would be cheaper than 5600 unless they've lowered the price as well.