Which Macbook Pro Hi-Res screen is better for editing photos?
This is 80% what calavera already said, and I'm only posting it for a few additional details that wouldn't fit into a comment. Translation: Accept calavera's answer instead, if you're thinking about accepting mine.

Glossy is generally considered better for accurate color gamut (I believe that's the proper term). Closer to "real life", full representation, etc. If you're professionally serious, I would still recommend picking up a color calibration kit, but you'll likely have less tweaking to do for accurate representation.

Anti-glare is exactly what it says, and exactly as calavera put it, it doesn't reflect the sun, room lights, generally light sources nearly as strongly, so you can use it nearly anywhere without having to re-orient/re-angle yourself. But, as a side effect of this, colors are dulled, a bit washed out.

Photo editing, video editing, anything that is geared for visual presentation pretty much demands a glossy screen, as opposed to something with just a visual interface (surfing, audio editing). Of course, with Apple making the anti-glare the "special order", it's there for people who need it, but otherwise people get the better choice in the majority of situations, by default.