I'm pretty sure the 154 proto has the equivalent effective edge to the 156 SL (or whatever the wide version is called) you can look up the stats on the site if you really want.
The Proto does feel lighter than the Sl in the hand. with bindings, boots and on your feet you'd be hard pushed to tell the difference.
You would probably feel the difference in dampness if you rode some lumpy snow on one then swapped to the other, if you pick one and ride it. I suspect they'd be too close to wish you had the other.
Buttering etc, both fine no problem. Though more effort than the Evo for sure though.
Power, both fine. Apart from 1 day where the power seemed really sticky and it was hard work (on the SL) all other pow days on both proto and SL have been zero effort. (though switch pow riding on the SL is more difficult (but still doable) - assuming you retain the standard setback).
Sl slightly better for freeriding and fast piste. Proto slightly better for playing. I believe the sidecut is slightly narrower radius on the Proto, so tight turns are slightly more it's things than drawn out ones. But you're talking nuances only. I'd suggest going with the one you like the look of more if you can't try them.