I just received my Blizzak LM-60s from Tirerack.com (sponsor!). I mounted them on 18" wheels so I can keep my sport package wheels with the same tires on and just switch out the entire wheels.
If you get all four wheels and tires the same size, you will be able to rotate them when the time comes. Winter wheels have more tread wear so this could be important if you are trying to squeeze value out of them.
If you get a more narrow tire, they are actually better than wider tires in the snow. Wider tires create more surface to push the snow and you effectively have little plows = not good. Narrow is better for performance.
Z drivers have had great success with Blizzak LM-60 and Michelin X-Ice 2 tires, both available for your 18" OEM wheels.
Narrow and square setup = better performance.
Wide and staggered setup = better looks.
I went with better performance..because... I don't want to crash. Simple as that.
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Pearl White 2010 370Z Sport, Stillen CBE, tint, K&N drop-in filters, GT-R start button, Kenwood DNX-6980 with backup cam, LED bulbs, Raceseng shift knob.