MWEB and Web Africa are independent of the big bandwidth providers in previous years. They both rent their own "pipes" from Telkom and then push it through their own international/local capacity.
MWEB uses Seacom with fallback on SAIX (run by Telkom). Web Africa uses SAIX exclusively although they have said several times they might look into Seacom as well.
IS runs mostly on Seacom and Satellite now (AFAIK they've ended their SAIX provided bandwidth).
Back in the early days of the internet, it depended where you stayed on which international bandwidth was right for you, depending on landing points of the cable systems etc. Nowadays there's no real difference, although you have to realize that if you're on SAIX based bandwidth (ie, not MWEB, Web Africa but Telkom) your ping times between gaming servers locally hosted by IS will be huge. However with MWEB's open peering agreements it's more or less the same as if you were on that specific ISP's bandwidth.
The downside to Web Africa is they only have one point of presence, and that is in Cape Town. MWEB has multiple POP's so latency between Cape Town and Johannesburg for example will be almost half (if not more than half) if that Web Africa can offer you if you were a Cape Tonian trying to connect to gaming servers mostly hosted in JHB etc.
Having been on Web Africa and MWEB, I can recommend MWEB hands down. Their latency to international games like World of Warcraft beat that of Web Africa's unshaped gaming protocols by a clear 100ms at least. And that was without using MWEB's free "WTFast" account as proxy/vpn. MWEB's stability also weighed heavily on my decision to move to them as there was quite a few lag spikes with WA and at a point getting 3k+ms lag. They blamed my line for that and since I was trialing MWEB's uncapped (to me, uncapped should have been ****tier than the capped of WA so decided to have 2 accounts), I quickly proved it wasn't my line or over-congestion of my Exchange.
In previous years, the shaping policies were decided by SAIX and IS, nowadays, the shaping policies are decided by the ISP (MWEB/Web Africa. Other ISP's merely resell IS or SAIX). Web Africa is by far *NOT* unshaped. They still shape all their traffic (but not to an extreme extent like we were used to with Telkom). 30-38kb/s on a 384kbps line average is what you will expect. And I think this is more or less what you will get with Web Africa as well.
The reaction/response time by MWEB to correct a shaping issue is quite fast, compared with Web Africa where I had to spend several days on their forums trying to convince the team something is wrong with World of Warcraft and their shaping (which, in the end, they admitted that something was wrong on their side and their "vendor" was looking into a fix, something to do with WoW going a more p2p route)
I concentrate on WoW since that's my barometer when it comes to testing out new ISP's. Patch downloading/connecting/playing smoothly without getting booted etc.
MWEB doesn't have "half speed" policies like WA does, even though WA claims they only do that when their capacity is reached which is (according to them) rarely. In my experience with them since 2005, it's a hard and bitter pill to swallow between what really happens and what is actually happening. I've loathed MWEB since the iAfrica days, and vowed never to become their client, but with the uncapped revolution (yes I call it that), I haven't looked back once. Even if Seacom goes down, 15 minutes later you're on SAIX and happily browsing. (As long as you don't do hosting with them or Web Africa you'll be fine)
Another thing that impressed me with MWEB vs Web Africa was that MWEB's shaping is extremely well done. I can run torrents (downloading at full speed) and once I visit my favorite website or check my email, the torrent's speed is AUTOMATICALLY throttled down so I can get full speed on my HTTP and mail. This makes downloading torrents while I work quite nice since I don't have to worry about speed or stopping the download just to get something done quickly like I did with Web Africa.
Do please note It's going to be one HELL of an experience switching your ADSL line from MWEB to Web Africa just to be disappointed (maybe) by their service/speeds.
What I would suggest you do is get yourself an uncapped account from them (just an account, not the line) and test it out. Your ADSL line, even though administered by MWEB, is still Telkom (all of ours are) so it doesn't matter or have any negative affect if you choose to use other ISP's on the line.
I think this will help you decide which is best for you going forward. That way you can get a taste of what WA has to offer without having to spend a month/two before getting your line moved, and then have to repeat the entire process just because they failed to live up to expectations.
It really does depend on what you use the internet for. I'm happy to keep recommending MWEB. The few extra bucks won't kill you
Take a look at our new prices for our capped accounts 
