Hello 123Liz
I can't give an exact answer to your question, but is your hotel far from Gib. and do you expect to do some sightseeing along/around the Malaga to Gib. strip/inland?
My wife and I are flying into Gib. a fortnight later than you (landing around noon) and are staying at the MacDonald Villacana Club resort (does anyone have any comments on this place) near Estepona.
We always have a car on hols as we are not pool-freaks.
I found alamo usually very competitve in the past and was quoted £126 for a 5door, AirCon, Clio (or similar).
I then tried DoYouSpain (I'd seen them on the web a few times - check their site); they have quoted £67 (yes sixty-seven) for the same thing, no extras and can pay on arrival. The slight downside is that their depot is just over the border but a rep. will be waiting (I've always wanted to be the subject of one of the held-up boards at the airport) to walk you the 200yards where you pick up the car (presumably then drive back to pick up wife and luggage) and away you go.
Crikey we could spend the 67 quid on transfer taxi costs and a couple of trips to a restaurant/bar.
It's worth thinking about.