Coming into my first winter in Mn and prefer to have two sets of wheels - 1 for summer and 1 for winter. 18" wheels / snow tires saves a lot of money PLUS softens out the ride over damaged roads. Any concerns?

 

Snow tires & winter wheels on the car as soon as the pavement gets to 35°.
18" is the way to go, from all accounts. No concerns except ruts in the roads and other drivers.
Any reason you're thinking about snow on a day like this in the 'cities? Heat stroke?

 

Have an '09 C4S in Saint Cloud,MN and use 18s for my snows.
This is my second 911 and I drive it all winter - FUN in snow!
As mentioned above, once temps are in the high 30s to low 40s, snows go on and stay on until sometime in April

 

18's are probably best but you can get winter tires for your 19" rims. Honestly the roads get so bad by spring that 18 vs. 19 wont matter. Blizzaks, Scorpians, Alpins and Dunlop M3's will all work great, but I see the most cars with Blizzaks.