As someone who's gone through both formal and informal rush as a rushee, I'd like to offer my insights from my own personal perspective.
Formal Rush
Positive Aspects
1. It's a lot more exciting. The rush of parties, house tours, etc create a thrilling atmosphere in its compressed state.
2. When you're rushing with the same group of people, you can see where they're leaning towards and get their thoughts on things. This is useful in that it a) gives you other information and pairs of eyes to consider perspectives of and b) if they're looking like a strong mutual match for one fraternity or the other, you can take into consideration if you'd like to be a fellow pledge brother with them in that fraternity.
Negative Aspects
1. You've got to be fast. Fast in evaluating your choices and faster still to make a good impression. This is really where the limited time factor comes into play and paying attention to signs. We've all discussed on here how some great PNMs slip through the cracks. That's something that can happen in formal rush.
2. Related to #1, even if you don't slip through the cracks, you can still end up pledging a house that you thought was the right fit for you, but after getting to know them more during pledge period, you realize that your initial impressions were wrong. This leads to either your de-pledging or initiating into a chapter you don't feel at ease around. In the first case, your chances of finding somewhere else have taken a huge hit. In the second, you get less than what you hoped for out of the fraternity experience.
Informal Rush
Positives
1. You have time to get to know the fraternities and they have the time to get to know you. This ensures the highest possible chance of mutual fit, which is the ultimate end goal in any fraternity or rushee's mind. (or it should be)
Negatives
1. It's frankly nowhere near as exciting as formal rush to me. Now some people will probably disagree with this, but it's my opinion.
So they both have their positives and negatives from a rushee's perspective.
