The big question mark about WebAfrica is their network performance after you hit the rolling soft-cap. As open as their policies are, this part of the deal is as murky and shrouded in mystery as everyone else's. Who knows - the usage patterns of you and the other 3 might suit their soft-capped network. It might suit Mweb's ... you ideally want to try both, and if you try Mweb first and it's not for you, you'll have done a ton more paperwork than if WebAfrica's didn't suit you.
The absolute worst case scenario is that you have a month of uncapped internet that frustrates you a bit.
Even better - see if WebAfrica will give you a test account for a bit, I remember a WA rep volunteering that on another thread here
As a matter of interest, what's the ISP of that account? Hit speedtest.net - that usually has a fairly good track record of guessing the account provider. I'm not sure if IS/Altech resell accounts to the point the resellers can attach their own whois information to the IP pools they're assigned.