Some might suggest that "you just know." I did. But that doesn't always happen. Some suggestions:
Take a look around at the other PNMs at each Pref party. These will be your sisters and the girls you'll spend a lot of time with. There might be some overlap in the crowd, but think about the other PNMs you've met going through Recruitment. You might have clicked with some and clashed with others. Look for those girls during Pref.
Think about which sorority is "the real you." Where will you be the most comfortable hanging out in your pajamas eating ice cream? If you love both but are intimidated by one, you might consider going with the one where you feel more relaxed.
If you still love both equally, maybe you should look closely at their philanthropies. Perhaps one is closer to your heart? You might also read their creed, open motto, purpose, history etc. Something just might click for you.
Still a draw? Then don't worry about it. You'll probably be happy at either one.
Good luck!