I was in Tanzania this summer( Dar and Zanzibar). Had a good time. Zanzibar has a nice combo of history and tourist type stuff to do. Dar is a fast developing city that I enjoyed but I think I had an advantage in knowing people who live there. Not sure if you'd be able to get to know the city well otherwise.
Regardless of where you decide on, don't limit yourself to one country. I was working in Zambia and took the train to Dar Es Salaam:
http://en.wikipedia.org/wiki/TAZARA_Railway
Pretty unique travelling experience, that allowed me to see the interiors of both countries. It wasn't easy but was a trip for the ages for sure.