Well, it's much easier to buy an external display than to return one. My advice - see if you can get used to the 17" on its own. If you can't, in a couple weeks/months just get an external display. It also depends on your needs. Me, I love having an external display (I imagine I'll love it when my new MBP 17" arrives too), so that I can work on one screen and have a full-screen iTunes thing going on on the other screen. 17 inches is a lot of screen real estate, though.