Also I'm unsure about the hooded version or the hoodless with the shorter legs. Any comments in comparison? Does the hood work ok with a helmet and do the legs + snowboard socks become a problem or it better to have the shorter legs?
You absolutely want the one with the hood.
It's thin, you can wear it with snowboard socks just fine, and it fits under a helmet and/or toque no problem. I ride without the hood 98% of the time, but on cold high wind days I put it on and it acts a good neck guard since there are no seems.
For sizing, I'm 5'11 and 180 lbs, most shirts I would go with a medium or a large depending how it fits, and I wear a large ninja suit and it fits just right.