I think MTN Zone is a must on topup
http://www.mtn.co.za/FindaPlan/ContractAndTopUpPackages/PackageOptions/Pages/default.aspx
There it shows the call charges for topup and under the table says zone cost R2.50 mtn to mtn and R3 mtn to other.
Over the weekend and at night you get very good discounts even 100% sometimes over weekends. Its charged at per second billing so it pretty much a must for me.
If you choose not to use MTN Zone you will be billed on a per Minute bases and that can get VERY pricy IF you call over weekends and at night especially if you phone MTN to MTN.
Its a really tough choice but for me I will go with MTN Zone. If you are not sure just switch cell info display on or broadcast messages in your message setting set it to channel 50 to see what kind of discounts apply to what times. It will make the decision a bit easier. Although I think you can switch between mtn zone and and the topup rate quite easy.