Ben... I'm not really sure that's all that responsive to the question.....
Cities in Flight... To be honest, though I quite like the set, and think they are wonderful stories in their way, they are rather dated, and not among Blish's best work, I think. If you're heavily into Golden Age sf, then by all means, go for it. If not, then I'd put that one lower on the list, and suggest going for
A Case of Conscience, or some of his story collections.
As for
Stand on Zanzibar -- a difficult novel, in some ways, due to its unconventional structure, which would allow it to be read in several different ways; nonetheless, I'd say a brilliant novel, well worth reading. It may take a bit of getting used to, but it's well worth the effort. And, if you're interested in such things, it's part of an "ecological dystopia" trilogy:
Stand on Zanzibar,
The Jagged Orbit, and
The Sheep Look Up (the last having been called one of the 100 best horror novels because of its presentation of the grim ecological scene). Now, you don't have to have read any one of these to enjoy the others, but read together, in this order, they each build on the previous ones thematically, making the final volume even more powerful in many ways.
So, if you had to choose, I think I'd pick the Brunner as the better book, in this case, though I do think
Cities in Flight worth reading -- just not as much so as some of Blish's other work....