It also depends on whether you want country-wide coverage or only in areas where 8ta has coverage (in the main cities). 8ta provides great value for money, but if you are looking to use your connection in smaller towns or on the road, MTN is a better option. MTN uncapped is also a great deal obviously.
Good 8ta signal is very area dependent. I read mostly good reports online, but found mine slow and intermittent. If that's all you can afford it is useable though.