I just bought a set of OEM 18 takeoffs from a fellow forum member. I'll put winter continentals on them (235 front and 265 rear) leftover from my last car, a 997. Narrower is a bit better for driving in snow, but the large majority of days are just cold not snowy and the std size rears will make for better driving. The true benefit of winter rubber is the compound and tread pattern.
"Accept the advice of others only in the rarest of circumstances." Beethoven
present: 08 E90 M3 / past: 2007 997 C2, 2005 997 C2S, 2005 Audi S4, 2004 Volvo S60R