Thanks, everyone!
The general opinion is to prefer B over A.
X is a subtopic within computer networks. Obviously, A has profs working on computer networks, distributed systems and related stuff - but not on X.
In general, I agree that I should not be very very specific to X and be ready to expand. I don't mean switching to different field just because A doesn't have enough research going on in X.
Agreed. But, its not about #4 vs #16 according to some XYZ Rankings and I know it doesn't matter much. I put "Top 5" vs "Top 20" just to give a general sense of the reputation of the places. And somehow, the reputation of the university is there in the back of my mind.
Ok. So, do you think one is better than the other?
Also, how much do the collaborations of the professors across univs matter?
B collaborates with top researchers in X and related area. A - well, not as much. I see some of the students at B spending their summer with top researchers and have a lot of joint publications/projects.