In short:
Vodacom
If all your contacts are on vodacom, and you only ever call them during "off peak", vodacom is cheaper.
MTN (EDIT, lets at least try to get this right)
If you recharge on thursdays, and for that thrusday only, you get 50% back* - meaning all calls are 50% cheaper.
This offer only lasts until end of this month only
You can only spend the "50%" back MTN to MTN calls
Joining MTN Zone (what ever that is**) might get you 100% discount to another MTN number
*Even with 50% more airtime to other MTN numbers, the price is still more than R1 per minute by my shoddy calculations
EDIT:
**When activating MTN Zone the package in-bundle and out-of-bundle rates will no longer apply. MTN to MTN calls will be charged at R2.50 and MTN calls to all other local networks will be charged at R3.00.
Otherwise, CellC is cheaper.