PhD programme dilemma: Doing it in a good faculty with strong supervisor or weaker supervisor but more interesting project?
Here are some good things to think about.


Research topics look very different when you dig deep into them.
Right now A's work might look sexy, but you can get 3 months in and
realize details that make you understand why it's not "hight impact". The reverse can happen with B: when you
actually start doing it you may find it more interesting than you
thought (this is quite common!). The fact that B's research
generates so much buzz means I think it's likely that when you learn
more you'd find it more interesting, or you might not.
Your Ph.D is a long hard process where you will have plenty of doubts. I personally believe the only way to keep your sanity is to be working on a project you truly love. I am saying that as a current student where right now things are tough but I'd never leave because this is the only thing I want to be doing, while I hear others who are almost ready to quit because it's a lot of work and the reward is quite far away. I know I'd be reconsidering what I am doing if I didn't love my projects.
You will do much better work and put more time into what you find interesting. 
When you say the research centre is tough/challenging, did you get any bad vibes from the students? Trust your gut instincts. They won't say anything bad about their boss when he's around. The only thing you'd hear on a visit is little sour notes dropped around. Pick them up and understand them. Look at the professor's CV. Are his/her students doing well after graduation? If they aren't, that's a big warning sign.
Have you asked B to explain to you why his/her work is interesting? Sounds silly, but that's probably the most common thing B has to do. It may totally change your perspective.