https://secure.telkom.co.za/today/ucm/
This is a link to the telkom coverage page one thing to note is their roaming is on Mtn's 3G and 2G network which in my oppinion is Legendary as the 3g Speeds are pretty decent
https://www.cellc.co.za/cellc/coverage-map
This is Cell Cs coverage map, They also offer roaming through vodacom and they have come to an agreement to use mtn's network in areas in which they do not have coverage I am not sure if this has been implemented though.
In my oppinion they are both Much of a Much, I use Cell-C LTE-A and i constantly get above 65Mbps (Drops to 40Mbps) during peak.
And i also have a TM sim card that gives me constant speeds above 20Mbps in Gauteng