How is this even a question?
I guess it depends on what kind of graphic design you are doing... but seriously. If you are a professional graphic designer, this is not even a question.
Mac Pro, all the way. Here's why.
#1 - Storage Capacity. I want to live my life without entrusting data to drives with crappy made-in-China data interface &/or ac connectors on the back. Find me a drive that does NOT have EITHER type of crappy connector that will fail on you. Tell me how it's better to have multiple external drives with cables draping everywhere, or to spend $400 on a Drobo that *might* just eat all your data, compared to rolling with drives that are safely inside your machine itself, covered under your AppleCare if you added it BTO ... ? Remember, this is the machine you use for WORK.
#2 - Color reference monitor. Wait, what? Yeah, the iMac has that disgusting glossy screen which will probably drive you to purchase a nail-gun and shoot yourself in the eyes after using it for very long... unless you are already blind or you just do not care about color accuracy. Sure, you could buy a second monitor, but seriously, working with two screens that are different brands and different calibrations (or rather, one that's worth calibrating and one that's not) is not all it's cracked up to be -- obviously these people have never actually done this, they just imagine that it would be "good enough." But I'm here to tell you that even just the presence of a screen that is not properly calibrated can throw you off by being in your peripheral vision for color matching. If you're gonna roll with two screens, you wanna get two IDENTICAL screens. On the iMac, even with the lights off your own reflection will throw you off to an extent for judging color, unless you are black (or paint yourself black) and dress in all black. The idea of "color calibration" on a display which basically has a translucent mirror glued in front of it is a freaking JOKE.

No, what you want is to get a Mac Pro and hook up a La Cie color reference monitor (or at least something with wide-gamut and a matte finish) so that, y'know, WYSIWYG!
#3 - You don't need any more reasons than the first two... but here's another one. Dual optical bays. So what? Well, here's so what. I put a Blu Ray in my Mac Pro and now I can back up 23gb of stuff to one disc. Again I dunno what kind of graphic design you do, but in my line of work I often have jobs that are way too big to fit on even an dual-layer DVD. Or I want to fit multiple jobs that were part of a huge project onto one disc for backup or transfer. Blu-ray with up to 50gb per disc on the dual-layer ones is freaking AWESOME and is the wet dream you expect it's gonna be! Plus you can directly copy DVD's without wasting the time to rip them to your HD first... I'm talking about the discs that customers send you media files on, not DVD movies.
#4 - Resale value. Much higher on Mac Pro.
#5 - Encoding things. Maybe you want to do more than just "graphic design" ... the Mac Pro has a lot more room to grow. Maybe you wanted to do some video stuff, encoding something to DVD. Maybe you wanted to get into 3D design. Whatever. Mac Pro = better.
Can't afford it? Get a used one that's still under warranty. Seriously.
-=DG=-