bottom tier: girls vs boys

Being in a bottom tier frat is probably worse because frats are responsible for throwing parties because sororities can't booze in their own houses according to the NPC. So there's really no pressure on sororitys to improve their social standing because they simply don't need to, whereas frat brothers do because they want to give off a good image to get people to go to their downtown parties. Also as a side note, keep in mind that tiers are real, but they're not as significant as people make them out to be.

Honestly, it depends on what you are looking for. I tend to dislike the larger, more crowded parties, so I have a great time at the less-populated bottom tier parties. Being in a frat, I think it was great because I could decide when I wanted to party and how hard I partied. I feel like sorority girls would have less flexibility in this regard (they wouldn't easily have the option to party on a Tuesday).

Also, if I were in the mood for one of the packed parties, I would just hit up some friends in some of the more popular frats and they'd let me in, so there was no huge downside to being in a bottom tier frat. Girls probably wouldn't have to worry about this either.

In terms of mixers and supers; it's probably pretty similar for frats and sororities as plenty of people wander around anyways. For formals; it's dependent on the individual to obtain an invite, or extend the invite.

TL;DR
It's opinion-based. Bottom tier frats have more flexibility than the sororities, but by no means is it bad/boring.