There are also some reasonably priced all season tires that would fit the 18 inch wheels. Obviously some of the handling would be compromised by the swap but I think in Maine (I'm in Pa which can be just as snowy during a rough winter) requires the compromise.
Don't quote me on this because the line between dealer-babble and actual fact can be a fine one, but I believe that any switch from 18 to 17 and so on requires a new wheel alignment. For 500-600 bucks you could just get the all seasons mounted and be done with the problem. Plus you'd keep the good-looking wheels year round.
Good luck,