Will you feel like haggling when you get off the plane?!
In my experience, you never get as good a deal at the desk as online. If you don't pre-book & the smaller (cheaper) cars are all out, they'll only rent you a bigger one at the class rate. If you pre-book & all the cars in that class are out, you get a free upgrade! Works almost every time, in all countries (or it has for us) - last week in France we got a Class I for the online special price of a class B! When we pre-booked with Hertz in Aus, we paid for a low-spec Getz & got an Avensis with air-con, CD player etc etc - every time!
Good luck
S