Basically, what I meant by that is this: in formal rush, you basically have one chance to make a good impression. If they don't like you at the first rush party, they cut you -- and often at the first rush party, there is so much going on that you'll only get to talk to three different girls for five minutes each, and there's not much time to go beyond the basics -- where are you from?, what's your major?, what classes are you taking?, etc. During informal rush, you will usually talk to more girls, for a longer duration, and about more in-depth subjects. Also, lots of times, sororities won't start cutting girls until after a couple parties in informal rush, because they want to have a chance to get to know the girls as well as they can -- so it benefits everyone. You have a lot more chances to let them get to know you.
(Don't get me wrong, I'm not trying to be negative about formal rush -- it has its benefits too. I joined through formal rush, it is TONS of fun, and I wouldn't trade that experience for anything in the world. But if you think you're the kind of person who would feel more comfortable in the informal rush environment, you'll probably do better with informal, just because formal rush can be so intimidating!)
I think there may be a Radford student or two on the boards; maybe one of them will respond to this post.
Good luck with rush, and make sure to ask if you have any more questions -- there are tons of people here who LOVE talking about rush, me included, so don't ever hesitate to ask us anything.
And make sure to stick around and let us know how rush goes this fall!