I believe Air Tanzania is a partner of sorts with South African. In the current *A configuration, South African would be convenient for the trips to southern Africa you mentioned, but in the discussions of which *A FFP to join, SA never comes up as one of the top 3.

I suggest checking the charts in the stickies at the top of this forum, plus a couple of long threads discussing the pros and cons of the various FFPs. It seems like you should get *G on just about any airline with your travels, so lounge access is given, but beyond that it goes down to personal priorities.

All the best for the big move!