Delurking here - thought this was an interesting thread.
When I went through rush there were 4 national sororities and a local at my school. The local didn't participate in formal rush, but held their own rush the following week. (Got their own rho chi and everything!) The nationals were huge (total was 80, quota was usually around 30-32), the local had just gotten started and was very small (7).
I went through 2 rounds of rush before I realized I wasn't really comfortable joining such a big group. As an only child the idea of suddenly having 100+ sisters scared me
Plus I thought I'd get lost in such a big group, and not really be able to make a difference to the chapter. So I dropped out to rush the local, joined, and I'm glad I did.
(The local later went national w/ AEPhi - hence the username)
Anyway, if the local hadn't been there, I probably would not have dropped out of rush, and would
-- edited: would probably have joined one of the 2 sororities I went to for 2nd round parties.
I don't know much about the nationals that weren't represented at my school, so I can't speak to that.