SoulSista,
Although I am not a part of a BGLO, I do understand where you are coming from. It's kind of two sided. ON the one hand, there are some people that you just couldn't see yourself bonding w/ or doing service with or what have you. Sometimes these people are so terrible that it DOES turn you off from their particular organization. This is what happened to me. So I'm feeling you on this. People were like "well you are pledging an organization and not just this chapter". That is true, BUT you are going to have to b around these people in the chapter for how ever many years after you get in (if you get in). So that was my thing. If you honestly can't see yourself being around these group of women for however long, then I would say go w/ your heart and if you feel more comfortable w/ the women of the other sorority, then there you go...you have your answer.
Now on the flip side--Just like AKA2D 91' said, the organization IS indeed a lot bigger than the chapter in which you are seeking membership with. Although these women may be "catty", they are hopefully, only part of a handful that act this way. These women should not reflect your outlook on the WHOLE organization. We all know that every group always has a few (maybe more) bad apples. Feel me?
So the choice is indeed yours. To me, if this particular chapter of whatever org is just NOT reppin the org to the fullest (i.e. not doing service, making you feel like you are beneath them, etc) then I don't think I'd want to get involved w/ this particular chapter. But perhaps, maybe you could (should) wait until you graduate and go grad/alumnae. If the org is really in your heart, then I think you would want to either go ahead and pledge (hey, maybe they are testing you) or if their attitudes really are just that terrible, then maybe a grad/alumnae chapter will be more suitable.
Sorry this is long, but I do think you really need evaluate why you want to be in a sorority and what do you feel you will get from it, etc, etc. This is a tough decision, but as my pastor always says "just pray on it".
Good luck and I hope I helped somewhat