This subject was touched on before...check out this thread for more advice:
http://greekchat.com/gcforums/showth...local+national
Here's my $.02:
First of all I have a lot of respect for locals (we have three on my campus), but when you're in a national GLO, you have the support of HQ and you also have a brotherhood/sisterhood that extends to other chapters. When you're in a local, you don't...and if you begin to have problems within your chapter, there isn't that larger connection to keep you from walking out.