I have an iPad 3 and a Laptop and on both I use about 3GB each per month. I am currently using Vodacom Prepaid Data, but I find its a lot more expensive than just getting a contract with a fixed number of GB per month.
I used MTN on my iPad when I got it and thanks but no thanks, poorest 3G coverage and instability ever. Vodacom seems to have good coverage wherever I go and seems stable enough. However, Cell C has a couple of good packages, but I don't know how strong their 3G infrastructure is compared to Vodacom, and what the coverage is like. I must however say that on my BlackBerry I noticed I have 3G coverage with Cell C most of the time. Pity though that Cell C Voice Services are completely useless, which lends me to give second thought to getting into a contract with them for Data.
I am more concerned about the quality of the 3G connection and signal strength, than the actual price at the end of the day. You get what you pay for... right?