Choosing a mentor

I'm a first-year PhD student in neuroscience. At my program, students come in without a mentor and do three lab rotations before choosing a "home" for their dissertation research. I'm on my third rotation now, and hope to finish and pick a lab in the next month or so. I'm having the somewhat pleasant problem of not knowing which lab to choose, because I like two of them.

Lab 1: PI is an assistant professor who's been with the university for about four years. She also taught one of my classes this semester, so I know her well and like her a lot. No students have graduated from her lab yet, but there are a few older students in the lab, and another member of my cohort just joined. The people are nice, although I'm not sure I'd become best friends with them -- they're pleasant to work with, friendly, welcoming, helpful, and share out the crappy lab chores in a fair way. The research is slanted more toward basic science (I tend to skew more translational), but it's still a topic that I like studying and it has some clinical relevance. One of their collaborators is a guy from my second choice PhD institution, so if I joined this lab I could get the best of both worlds, working on projects with him even though I chose not to attend his program. I feel like I have a pretty good grasp of the techniques they use, and while some are annoying, they're all things I can deal with.

Lab 2: PI is a tenured professor who recently came to my university from another institution. She has mentored graduate students before, but currently has none. Since she's new, I can't get dirt on her from other people in my program, but she's nice and the people in her lab seem to like working for her. Other lab members are great -- we socialize outside of work and have a lot of fun in the lab. They've also been awesome about teaching me new techniques and answering all of my questions. The research is more along the lines of what I'd like to do, although the specific disease model they study is something I'm not very familiar with (nice because there aren't many competitors, but less interesting than a more 'sexy' topic). This lab uses a ton of different techniques, some of which are totally alien to me. I feel like it'd be a good learning opportunity, but I don't quite know what I'd be getting into if I sign up (I'm still mid-rotation here, so this may become more clear in a few weeks).

I guess it comes down to -- Lab 1: love the PI, less psyched about the research topic but like the techniques, and the lab is "unproven" in that the prof is still a newbie. Lab 2: less sure about the PI (but still like her), think the research is fab but not sure about the techniques, lab is "unproven" in that nobody here really knows these folks yet.

My current plan is to take Lab 1's grad students out for coffee and pick their brains, ask the PI from Lab 2 if I can have email addresses of former students to do the same, and talk to both PIs in detail about what my dissertation project(s) would be if I joined. I'm not sure how to weigh these factors and make the best choice, though. I actually think that they're both good choices, but... any advice you guys can offer about your own experiences would be great, especially if you can think of good questions to ask the PIs and their students that will help me make up my mind. Thanks!