I have rented directly from Hertz a few times, UK and abroad and have no bad words to say about them.

However look at this recent thread and judge for yourself:

tripadvisor.co.uk/ShowTopic-g186525-i108-k61…

... a poster new to TA who posted about Hertz on three forums and hasn't been back to add any more detail. All I'd say is be very careful about checking any car when you pick it up, and be equally careful about what the basic and CDW insurance do or don't cover.

I have not used carrental.com, the're simply one of many broker/agents. I doubt i would make any difference to Hertz if you book via a 3rd party, but personally I prefer to deal directly because the situation is simpler if anything does go wrong.

Hertz and Avis are obviously the big boys. You might consider Europcar or Enterprise; both operate from the airport or in town, I think.