I think 18's are best for all of the reasons posted above but you can certainly get it done with 19's. I would have opted for the 18" OEM wheels and upgraded to an aftermarket 19" wheel for the summer personnally. But since you already have the 19's, you can use them for the winter if you desire. Another option would be to sell (or trade) the 19's you have and get some 18's. If I were to get snows today, I would either buy OEM 18's or 18 x 9.5 apex wheels. In both scenarios I would run 245/40 or 255/40 on all four wheels. When I used to live in MN I ran the OEM 18's with 245/40 & 265/40 snows. They worked well but I would lose traction in the rear if the snow was greater than 6". I don't know if a narrower tire would have helped me or not. I was pushing a ton of snow which leads me to believe that I would have been screwed no matter what. The other thing to remember with narrower tires is you often have less rim protection - tire could be stretched more or lower side-wall height. I would not be interested in running 235/40's on my car for that reason. That is not a very tall side-wall. I hope this helps!
'13 SSII M3 Coupe - KW Clubsport 3-way (Eibach Front Spring Conversion), Akrapovic Evolution, ESS Tune, StopTech Trophy, IND Euro Front Bumper and Gloss Black Mirror Covers, TMS Studs/Lugs, BMW Performance Carbon Fiber Spoiler/Interior Trim/Center Console, BMW Edition Grills/Gills, BMWPedals Extended DCT Paddles, Mode Carbon GT4 Splitter, Advan TC3 (18x9.5/10.5), BBS E05 (18x10/11), G-Force Rivals (275/35/18, 295/35/18)