Visiting Nantucket or Martha's Vineyard?

I work for a company that offers free flights on their planes and I can bring a friend along with me. The station I work out of only flies to Nantucket and Martha's Vineyard. Anyways, I'm planning on taking a girl over to one of these islands for a date and I'm not too sure which one to go to since I've never been to either one before. I was thinking of heading over around and returning at night. So I just wanted to know which one would be the better choice for a short trip, restaurant-wise and beach-wise i guess.