I've always hired cars from the airport when came to Cape Town on holiday. Hiring cars from the airport is slightly more expensive than other locations, although it was convenient. For the last 3 times I booked though Europcar and found them to offer the cheapest deals for hiring cars in Cape Town. I always select the most basic vehicles - a super-mini with no air con. With the last 3 booking through Europcar they have always booked the car through Budget. I've never seen any of the 1l or 1.1l suoer-minis (Chevy Spark, Hyundai Atoz) in the Budget parking area and all 3 times they have given me a 1.6l Chevy Aveo with air con and CD player. Some of the cheaper car hire places use Citi Golf cars (basic MK1 Golf shape with no air con).
International people usually get better deals than the SA locals when hiring cars, so it is better to book in advance from the UK rather than booking something when you get here. When I moved here I hired a car for 14 days for the time between me arriving in Cape Town and my car arriving at the port. I was then told that it would take 3 days to empty the container ship and the car needed to be inspected by customs, so I needed to extend the hire period by another 5 days. Because I had pre-paid the car, Budget then wanted me to sign a new 5 day contract for the car I was using and pay in the local currency. I ended up paying as much for 5 days as I did for the previous 14 days! When I queried this they said this was their standard rate - I was on a very low promotional rate with the original booking through Europcar!