A.S.
You will want two seperate things, one for car use that is thin (no heavier than fleece)...those are safe for carseats, anything thicker compromises safety, and then a big thicker outfit for the snow. We do a one piece for everyday in and out of the car (my son is 21 months). I haven't gotten an outfit to play out in the snow really yet, but I may just use the same outfit with layers underneath. I would imagine a one piece might keep them warmer outside (no way for snow to sneak in) but a two piece might be more convenient and easier to put on (I know getting the one piece over the shoes etc is difficult).