I use both interchangeably, although I've noticed that when I'm discussing it with people I perceive as being on a higher plane than me, I tend to use "I have bipolar disorder" rather than "I'm bipolar". It's like I need to separate it from me somehow, while it's no big deal around family or close friends because they KNOW I am (have) BP. The only thing I really don't like is when we're called "bipolars" in a generalized manner, like "All bipolars have trouble holding down jobs".

Personally, "being" or "having" bipolar doesn't matter to me because no matter how you state it, bipolar illness is a life sentence and in most ways, it really sucks.