This sounds like a really easy choice to me, unless there are other factors you've left out from the list (Funding?). But based on what you've listed, School A gives off a whooole lot of negative vibes compared to school B. Since your life is so defined by your day-to-day research activities, I think it's definitely crucial to 1. Be comfortable talking to your PI and 2. Enjoy hanging out with your lab mates. The biggest warning sign for me is the "unusually high" drop-out rate from the lab. People coming into Ph.D. programs are usually very committed and serious about their research -- it's going to take a pretty miserable experience to completely drive them away.
If you're still torn, I would definitely try to get in contact with graduate students from both of the professors' labs. Seems like a good way to get the low-down about what your day-to-day life would be like.
