I took the RS tour last year, and the bus ride through the Shankill and Falls Road areas was interesting. We made one stop, at a mural along the separation wall. The guide (Susie Millar, excellent) had grown up in Belfast during the Troubles, so her description of the area, and other discussions in other parts of Belfast, were probably better than a non-Belfast-based guide would have done. I didn't do a Black Cab tour, but I think that tour would have provided much greater depth and detail, as well as more stops for closer looks. So if you're especially interested in that area and material, and have time after the RS tour, you'd do well to go with a Black Cab. In fact, your previous bus ride through the area would probably help you ask better questions and go into more depth on the cab tour, making it a better value for your time and money.
I left Belfast the afternoon after the tour ended, but had time for a visit to the Ulster Museum, which I thought was quite good. I went through the exhibits inside City Hall during free time on the tour, I guess the last afternoon before it ended. Both were well worthwhile. An extra day or two in Belfast could be well spent if you have time.