Despite what people on here say, people who use the screen professionally (that doesnt include me of course) say the anti glare is better for serious work.
The glossy is awful in my opinion, even indoors you get horrible reflections everywhere which is really distracting. I have briefly used the my anti-glare outside on an overcast day and it was perfectly usable, my sisters glossy MBP was just ridiculous.
Some will also say the glossy screen gives better colour blah blah blah, but again, pros have done calibration tests and the anti glare is pretty much exactly the same.
Just search for comparisons between the two.
Personally I would never get any screen that is glossy, Apple have really dropped the ball when it comes to professional use displays, which is why the 27" and 30" LED displays get such awful reviews from people who use them professionally.