I'm not familiar with "In the Heart of Darkness." Do you mean Chris Peers' "In the Heart of Africa?" If so then I don't think you can go far wrong with that, though to be honest both that game and T&T provide excellent entertainment for Darkest Africa skirmishing, and I would find it hard to recommend one over the other. I have enjoyed them both immensely. The main advantage ITHOA would have would be the fact that it already has very comprehensive army lists worked out for Darkest Africa, whereas the lists in T&T are for more mainstream colonial theatres.
But if you already enjoy T&T, you could do what I do, and use the ITHOA army list book as a resource for building T&T armies.