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For snow tires, 17" will be sufficient. But if you have an extra set of 18" laying around, I say, hey, why the f ck not. Go with 18's if you got them. 17's if you gotta buy it.
If 17's, go with the 06+ GS 17" wheels. Those are affordable.
If it were me I would buy 19's for summer and keep the 18's for winter w/snow tires...But that's me...17's would be fine...you may find a good deal on ebay.