Onset is not a sailing harbor, although a great place to stop. There is a motley assortment of day sailors from a classic Herreshoff to a Capri 14. I've never liked cat boats but they are clearly popular around the Cape. I'll add my suggestion of an O'Day Day Sailor. The Rhodes 18 and 19 are nice sailing boats. A lot depends on where precisely you'll sail. My father-in-law moved up from said O'Day when he found the canal current a bit much for the centerboard.