One thing to consider is that it is considered very bad form to dump a cassette dosed up with heaps of blue stuff in any sort of toilet not connected to the sewer.
Horses for courses. If you have a black tank it needs to be large to keep you going while you search for a dump point (you can afford). Cassette can be more convenient (pun intended) if you spend a lot of time away from the main drag.
In Europe, Australia and South America, cassettes are in the great majority compared to black tanks simply because of the size of the RVs more than anything else..