Looking at MAC's Cork eye shadow and Urban Decay's Buck eye shadow in the pan it is hard to tell the difference between the two. They are both very good for use above the crease and blending out darker eye shadows. But, when using them there are differences in these two eye shadows.
When swatched, you can tell that MAC's Cork has more red in it than Buck does. The texture and color payoff is also very different. The Color payoff of Cork is excellent. It only took me a couple swipes to get the color achieved in the picture. The texture of Cork is very smooth and easily blendable. You can purchase MAC (Cork) eye shadow in any Mac store or the online store for $15 for the standalone eyeshadow, or $11.50 for the palette refill pan.
Buck, on the other hand, took many swipes of the product to achieve it's color in the picture. It did not go on very smoothly and is very less pigmented than Cork. Urban Decay does not sell single eye shadows of Buck. It is only available in the Urban Decay Naked Palette for $48 at Sephora, Ulta, or Urban Decay's website.
I know a lot of people like the color Buck by Urban Decay but in my opinion, Cork is a better quality eye shadow all around.