Renting at the airport means you get hit with a premium location surcharge, which was 28.5% last time I checked (Avis), if you have managable luggage catching the train or bus then hiring from a city location will save you 20% or so on the rental.

Yes, if you book online directly with the rental company you can cancel at no charge. So you can find the best deal now, and check back every month or so to see if there is a better deal and book that, then cancel original booking.