At my school - definitely sorority rush.
Fraternity rush is a total free-for-all. There are about 25 all-male fraternities and 5 coed groups. Back when I was there, rush started on Friday afternoon and fraternities started offering bids early Monday morning... that's not a lot of time to visit 30 houses in. So, you had to make use of summer rush and you had to have a good idea going in of where you might belong, before rush ever started... and you might unknowingly cut a house that would be a good place for you without ever having seen it. Plus, the first fraternity you visited was generally a random matter of which fraternity's members you happened to be nearest when rush started. From the fraternities' side, it's no picnic either... I know of one popular house where they would make a snap decision as soon as you walked in the door, whether or not to flush you immediately.
Sorority rush is more structured. There are only 5 sororities, so you can take the time to visit each and make well-considered decisions about which invitations to accept for each round. Aside from sorority rush, there are also those 5 coed groups, and 1 all-women's living group that's not a sorority, to visit.
I hear my school is going to deferred rush, though, where rush will be spread out over a couple of weekends. Hopefully that will make fraternity rush a little less hectic.
