As a greek, we will encounter situations like this on a daily basis. Even if it's just a look from a GDI on letter day. From personal experience, I must advise you to simply REPRESENT. Basically, whoever you come in contact with as a sister of XYZ, you create their impression of a "sorority girl." You can have massive backlash when they immaturely and naively bring up social fed cliches or you can adress the issue at hand in a mature and composed nature and actually educate these people.
As for your situation, if you calmly inform them of your chapter's atributes and make them realize that the greek system is only as strong as its weakest chapter, they will look like idiots.
Here's some points I usually make in response to GDI's statements:
You bought your friends:
"No, these are the people I would have found eventually as I have so much in common with them. Besides, not a penny of our money goes to parties and blonde hair dye(haha). We spend most of our time raising funds for our respective philanthropies...(Then talk about your philanthropy and make them see how much you care!

)
Sororities and Fraternities are all about partying:
Bring up study tables, test files, ISS and IFC sports, leadership opportunities, alum connections! Make them see what a powerful network the greek system is. Tell them why you joined. I joined because the greek system ecompassed everything I did in HS...the activities that got me into my educational institution! They can't bash that.
In repsonse to the girls:
Well, if you have the GDI boyfriend there, they will most likely be the center of his criticism since it seems these girls are rather shallow if they are 'comparing whose sorority is better." So don't worry. You just let things roll off your back and you will be respected.
Good luck!
pbpck