It's definitely a gut decision. When it comes time, you'll have a pretty good idea. And if you don't, it's perfectly ok to pace around and talk to some recruitment counselors.
I also think that it is a good idea to take a good look around at the PNMs who are returning to parties with you. While not all are destined to join those chapters, it's a pretty good indicator of the kind of girls you'll be spending a lot of time with (in addition to the existing chapter members). Not gelling with one or two isn't a deal breaker, but you should get an idea for who you'll be calling "Sister" for the next few years!
In my case, I came in really liking one sorority (ABC). I had a couple of friends there and just really loved the women I was meeting at each recruitment event. Surprisingly, there was another sorority that caught my attention (XYZ). I found myself really liking them and preffed them, too. After prefs, I was in a bit of a quandry. I talked to some other PNMs, made a list and thought about how much fun it would be to see XYZ on my bid card. Then I realized how sad I would be to not be an ABC. I ranked ABC (ADPi!) first.... and I got my first choice!
Enjoy the rest of your recruitment and best of luck!