Magic Ive previously ben an MTN subsciber for over 10 years and six months ago I decided to change over.
My main fear was coverage and data speeds.They were a bit unfounded but also sometimes founded so it depends where and how you use your phone.There are two aspects to consider 1 is cost the other is quality and in my case it was well worth it and a very good decision to port.You should decide for yourself though.
I found that where I work and live (vereeniging and surrounds) i used to get speeds of around 600 kpbs with mtn up to a max of 1,8 mbps when the network was quiet.With cell c i get average around 1,2-1,8 mbps and when the network is quiet it can go up to around 3,3 mbps.So the speeds on Cell C are generally higher however I would say MTN has better uptime,around once a month mtn data used to go down for the night in my area on cell c it happens around three times a month but thankfully only for an hour or two.
The signal Ive experienced in and around JHB itself has been excellent on Cell C also and the speeds are better than in the vereeniging area.
Where I do experience a problem though is when I go visit my inlaws in balfour(mp) where there is no Cell C 3G so it roams on vodacom edge and is largely unusable,call quality is great there on edge but I had to resort to buying a pre paid mtn sim for data.So if you do go to rural areas often I suggest you first check out the Cell C 3g coverage there specifically or get an MTN 3G pre paid card for those occasions.
Now that being said I have to say I wont go back to MTN as my main provider because of their excessive costs,while previously my MTN bill used to come out to around R520 with calls and data while on cell c i pay around R350 for the same usage.