cost wise airtel has cheaper bundles, but telkom has an *unlimited offer that airtel does not have. For quality it depends on your location, both are poor when it comes to wide spread connectivity, so depends on your location.
If signal strength is not an issue go for airtel if you do occasional bundles, otherwise go for telkom if you want an *unlimited offer, but be careful of late telkom has been having speed issues, been seing complaints on their twitter handle.
*unlimited for telkom has a cap of 6gb per day, if you exceed the speeds are throttled.