It depends on what you were planning the to use the SL for. I got the SL as a park board that was pretty good on the slopes as well. Initial reviews of the Cobra make me think the Cobra is going to be better than the SL for what "most people" actually do on the SL... ride groomers, get some air off of natural features, butter a little on the flatter slopes, and ride powder.
I say "actually do" because a lot of people believe they need a twin board to do any type of freestyle tricks... just like people buy an SUV or a pickup truck imaging themselves to be some type of off-roading tough outdoors men... when in reality they are mostly driving to the office and buying groceries.
While I do think the Cobra will be miles better in powder than the Proto ... I laugh at the idea that the Cobra is mainly a powder riding board... boards like the Never Summer Summit, Burton Fish, Prior Khyber (all of which I've ridden) are true powder specific boards... the Cobra shape is just more powder friendly than your old-school true twin. The Proto is not "bad" in powder (virtually no modern board is "bad" in powder now a days)... but it is not particularly good in powder either.
Remember, I'm saying all of this even though the Proto is the board I rode most often last season... it's just that I was riding in the park most of the day. It wouldn't be the board I would grab for free riding with friends or on a powder day.
+1. What he said. Just starting to test my Cobra here in Niseko. Loving the board so far, but it is clearly a (directional) all-mountain board, not a pow board. Although it is probably better in pow than 90% of all general all-mountain boards, it cannot touch my Fish or Sick Stick or other dedicated pow sticks (Powder Snake, Charlie Slasher, Summit, etc.) in the fluffy stuff.