I'm not a BB user, but I've heard more people complaining about Vodacom then about MTN, both here on the forums and people I know/met/work with/clients. This is w.r.t. BB only. But there are definitely problems on both sides.
In my experience, MTN's data network is superior to Vodacom's without a doubt. Vodacom has a pretty good network nowadays though - or so I've heard. I've only used MTN for the last few years, so my opnion of MTN would be biased. With regards to getting issues and problems sorted out, MTN is a complete nightmare to deal with. Sometimes it is impossible to get hold of the right people, and MTN's call center is probably the worst of all South African telecoms companies (save for Neotel, perhaps?). MTN is notorius for over-billing you, and then taking months if not years to rectify the issue, if at all!
There are several MTN employee's here on myBB who will help you out (unofficially), so all is not lost if you put in some effort and ask around when you have a problem. It helps to know this beforehand (well you do now) rather then spending hours on hold with the MTN call center only to be served by an uneducated untrained idiot (or rather well trained idiot??)
Maybe you should consider going prepaid, so that you can switch/port should you not be happy with the service of either provider. Choice is power. However, you'll have to fork out upfront for the full cost of a BB device. But it will probably be worth it in the end.
But don't listen to me too much.. wait for more replies from existing BB users from either side...
