The plus side here is with either board you will be happy.
I have both the Proto (157) and the SL (155) and love them both. Others have provided some points on the differences between the two but I do really think they are small nuances vs. major differences.
To me, the SL does feel more damp and handles speed a bit better. The problem with that statement is... the Proto is nice and damp, and fine at speed too! The Proto is a lot of fun to play on. It is not a "park board" noodle for sure but still a blast to ride. The problem with that statement is... it wasn't like I was bored riding my SL before getting the Proto.
So yea, with either board you will be happy but I do tend to agree with lonerider that based upon what you've said the SL wins it by a hair.