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18" wheels

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You might want to check to see how much space there is between the 19" wheels and the front steering knuckle. If you think you can squeeze 0.5" in there, go for it. The 19s are pretty much the smallest wheel you're going to get. Why do you want 18s? Cheaper tires?
 
Why would a higher profile provide better snow driving?
Less width to get higher ground pressure might be desireable though...

Because a tire with a higher aspect ratio will be narrower (if correctly sized). This makes for a longer contact patch, which will result in more forward/reverse traction. (Think track vs. tire)

Ground pressure is determined almost 100% by air pressure. Tire width has nothing to do with it, other than that the wider the tire, the less traction you will get on anything other than dry pavement (in that case it's the contact patch area that gives you the traction).