Really depends on what you want to do mainly, voice or data.
If data, then neither Cell C nor Virgin are really ideal. You know about Cell C already as a customer; Virgin offers super-cheap data for low-usage data customers (50c/MB) but still a very patchy service with plenty of outages and no high-speed connections, only GPRS.
Once you get to higher usage on prepaid, data bundles make the two main providers cheaper per megabyte (e.g. Voda and MTN offer 500MB bundles which work out at around 35c/MB -- even cheaper if you buy your airtime from a discounter like Pick&Pay, Jet, Pep etc. who knock a few percent off the price).
If I interpret the consensus from this forum correctly, Vodacom has a more universal high-speed coverage (3G/HSDPA) but MTN tends to score in areas without high-speed coverage, where they often have more EDGE than the plain-vanilla GPRS which Voda invariably offers.
If you're a high-end user, and putting aside the coverage above, Voda is probably the way to go if cost is a factor: unlike MTN, Voda offers discounts on bulk-buying prepaid airtime (a standard R1000 voucher actually loads R1100 of airtime), so it's effectively about 10% cheaper. Many users also say that MTN support is better, though that's not been my experience. In the end, you pays yer money and you takes yer choice!