blondebutsmart,
My understanding is that tiers are determined by how many women want to join the chapter that year. Sometimes it is very clear and part of campus culture (like an Old Row designation) but can also be a certain mix of sisters that make people want to be a part of them. Sometimes it's just a bunch of shallow but beautiful girls that all wear designer clothes and get smashed with the hottest guys on campus, and for some reason this is attractive to PNMs. Tiers change at most schools and you are much better off considering sororities that you feel you connected to.
I think sisters in "lower-tier" sororities often enjoy their experience more because "upper-tier" sisters may look around and think "ok, I joined this super popular house but I don't feel like I belong". The "lower-tier" knows that everyone in the chapter really wants to be there, not just because they won the popularity contest.
One more thing: if you've been reading other threads, then you know sometimes GC posters say not-so-supportive things. Don't let random people with too much time on THEIR hands (let's all admit it

) get you down. You asked a good question and for the most part have gotten great answers!