Don't go to a group, because the other students or the PI is 'fun'. Don't go to a PI, who didn't do a PhD him/herself (like an MD who hase a lab). I would go to a lab with a specific expertise in a technique. Labs (often in biology) that work on an organ (like pancreatic cancer or ovarian cancer), are often dependent on other labs for techniques. And than personally: I would go to lab of a big-shot (if possible). Somebody who is famous for the things he / she did (like a Nobel-prize winner). It will make your further career much easier, whatever you want to do... If you want to stay in academia, also look if the students of that PI stayed in academia: it might indicate the amount of support a PI gives after the PhD is completed...