Narrower is better for the snow but you end up with a lower profile and less rim protection from curbing. Both of which are bad. It's a balance and each approach has trade off's but I would go with 245/35 or 255/35 in the front and 255/35, 265/35, or 275/35 in the rear. My suggestion will happen to look better too. I don't like the site of narrower than stock tires on an M3. You live in NYC which sees snow but isn't it limited to only a few days out of the year? If that is the case, I would definitely go with the bigger tire sizes.
'13 M3 - Silverstone II - Sold
'04 M3 - Jet Black - BBS E88, KW Clubsports