In general, you get better winter performance from a larger sidewall (18" wheel is better than 19") and a narrower tire.

 

True, but porsche approves both 18 and 19s for winter driving. I've done both, had no issues whatsoever with 19s in the snow last year.

 

You can read above in my previous thread about the tyres Porsche is recommending...

I have 19'', and I would not go smaller or narrower- the looks are great + You really can do it through winter- believe we have the snow here!!

 

18 or 19 is really personal preference. Snow tires are available in both sizes (while they last). Generally speaking the narrower the tire the better for snow grip and for wheel protection. 18 x 10" rears with a 265 is still the best combo to maximize the snow grip. If you choose to stay 19 you can, just make sure you put snow tires on. Remember, it's primarily the compound, not as much the size that is doing the work here.