Heya.
CRMSNTiDEGRL, I went through formal rush and I actually didn't make it, when I went through my first time. I was tense and nervous, but going through formal helped me meet all the sororities, and find out what one was most like me. I remained friends with a lot of the KD's, and a year later, and after rushing informally, I am a sister in Kappa Delta! I'd say go through formal, and is you get into one, GREAT!
If not, then you've already met all the girls, you know what they're like. You can always rush the sorority you like informally later.