I was also under the impression that the SL > Proto, in terms of versatility and quiver-killer-whatever etc
The proto has the same stiffness rating as the SL, although the SL is a little damper and has a slight setback. These features may give the SL a slight edge for carving.
Also the proto has the bullet proof carbonium top sheet and is lighter.
Finally the proto has the blunted tips so its better for spins and has a slightly longer effective edge.
I have ridden both and they are pretty close IMO. Both very versatile boards that do most things very well.
The size you choose makes a big difference too. If I rode the 154 it would be more park, but the 157 is perfect for all mountain at my weight (for my preferences).