I generally prefer the flexibility, if the price difference isn't extreme. We take a lot of smaller, driving trips, and we both have family entanglements that sometimes come up, so we tend to make tentative plans and only commit when we get closer to the date.
Plus, too many things can go wrong when prepaying through a third party. I've had occasional issues with prepayments (or even reservations!) not being recognized initially by the hotel, problems trying to modify a reservation (even when allowed), etc.