Monitor to be sure. CrossfireX won't get noticed until you go into the 2560x1440 or higher range, so you're better off getting a larger monitor first. As an added note, my experience with CrossfireX was that it was hard to get working right depending on the game I tried.
I respectfully disagree, but with a caveat. I know it's not for most people, but if you have a 120 hz monitor, you've just doubled the effective number of pixels. If you like high fps then crossfire does help.
Having said that, I both agree that the monitor is the better choice, and that generally
you're right about crossfire for most people.