Smith does theirs so that you have to buy different ear flaps or the one the audio-ready one that comes with the speakers from the get-go.
If you're looking for it as a powder ski I would say go with the longer one for sure.
probably 178 Cm
I think you'll find the S7's a bit stiffer. This year's version features some metal sheets (Titanium I think?) in their construction while the keepers, as far as I know, do not. I think the S7's also have more rocker.
I have the 30 degree and I use a compression stuff sack that packs it way smaller than the non-compression one it came with. It works great.
If you are an expert skier you could probably go either way. With the rocker/camber combo on the S3 you'll get a much shorter turning radius than it would seem for the ski length. That said, if you ski a lot of trees and expect quick turning you probably want the 178's. At 185 if you ski a bunch of powder you'll definitely see a bit more float from the 186's. I have a friend about that size with the 178's and he wishes he got the longer ones.
I have a wool shirt from LL Bean and another one from Smartwool, both of which I've had for a while and I really like them both. Quality products.