The Proto does have blunted tips so the for the CT the 157 would be fine for your weight. Whether or not to get the CTX depends on what type of boots you are using and what angels you have your stance set to so what boots are you using and what are your binding angles at? If the CTX is the way to go then i would say the 158.
With the blunted tips the board rides a little longer. So the 157 rides around ~4 cm longer if i'm not mistaken. Same goes for the 158.
Edit* Also, what bigmountain said for sure.