I think that the majority of the members who ran winter tires opted for 17" wheels because there are more options to choose from.
The OEM rims (at least the 2005 SL) have a tendency to collect snow in the hollow part of the spokes, especially when traveling in deep snow. This throws off the wheel balance and can create one heck of a vibration until the snow is thrown off. I had read about the vibration on this forum so I was not surprised when I experienced it. After a mile or so the snow was thrown off and the ride returned to normal.
If I could afford a second set of tires I would get a set of mounted and balanced snow tires from the Tire Rack. This is what I've done for my cars in the past and what I do is run the snow tires for the winter and use the good set for summer.
Thats what I usually do...get a second complete set of winter tires on rims but the question is if it is possible to get 18 inch steel rims for winter tires. I used to have 16 inch for altima but I am not sure whether 18 inch steel wheel exists. Secondly if there are no 18 inch available, is it possible to downsize to 17's? I checked tirerack but the package they are offering come with alloy rims (17 inch).