I've always preferred Graphite more, as it gets out of the way visually (kinda like OS X's behaviour overall). That's what people mean when they say it's more "professional" if, say, you're doing graphic design work, having bright colours all over the place can be distracting and/or skew how you're looking at your piece of work. The same reasoning is behind their use of neutral grays, whites and blacks in their products (especially the displays). Not to say that's the
only reason behind it, but it's one of 'em.
Really, though, I wish Apple would create a theming system for OS X's UI. It's already obvious that the graphics can be swapped out at the click of a menu, and they could always provide guidelines (like how they have the HIG for app design) so we don't see as many neon nightmares. Hell, they could probably even incorporate the use of Core Image filters; they already use them for the menu bar, and allow devs to use them on the controls in their apps. I don't want this to sound like a ringing endorsement of Aero or anything, but Terminal looks ten times more awesome with
Blurminal installed.
(That, and even on my almost-four-year-old PowerBook G4, I can move the Terminal windows around in real-time with no lag, and the blur filter is applied to the background flawlessly.)