For early researchers, how does the ranking of a university relate to future career in high ranked universities?
Let me tell you what I told a friend who was struggling with a decision between a great department without people in his area and a good department with people in his area (including collaborators):

Go to the better school.  They're not in the same league, and you may have a chance in the future to get more people in your area.  You can still travel to the other school to collaborate, but you'll get much better students and postdocs at the better place.  You'll be in a better environment exchanging ideas with top people.

This answer of JeffE to a related question discusses the "better environment" aspect more.

He did in fact go to the better school, and afterwards was happy with his decision.  One thing he remarked was just because of the what university it is, they had top people coming in all the time and he got to interact with so many of them in a year, which wouldn't have been possible at the other school.

While I won't say this is universally what you should do (there may be other factors that matter to you here), I don't see much advantage of B, unless the people at B are truly top people in the area (in which case they might leave soon) that you want to work closly with, and wouldn't be likely to if you went to A.  Since you say University B is not research intensive, you may not have as much time as you would for research at University A (e.g., the teaching expectations might be much more per class, or more administrative duties).  Note there can also be tension in a department when part of the faculty is research active and part is effectively not.