I think your instincts on both are right. Peaks would look a bit out of place on a light grey suit, let alone a linen one. You're wearing a linen suit, because you're having an informal wedding. If a linen suit is appropriate, then so are notches. Obviously, wool would be more versatile than linen, but that doesn't mean you shouldn't get a linen suit. There are lots of summer days when a linen suit is useful.