Same here.. I have 20" with a 17" spare. Your 17" and 18" will have the same rolling diameter (or near enough), so it wont matter and like our other good friends that replied, you would only be using it to get home or to the repair shop.

The 18" spare is going to be a little heavier as well, but that is nothing to worry about.

Save your money, keep the 17" spare.