Stila on the left, CoverGirl on the right.(Sorry for the green blob in the second pic - it's my lamp. Also, my fingers are not red, the lighting is weird!)
As you can see in the swatch, the colors are similar, though Stila's is more gold, CoverGirl's more frosty. Also, the Stila shade of course has better pigmentation. However, on my lids I can't see a difference, so I'd say if you're lusting after Kitten, try out CoverGirl next time you're at the drugstore.
Buy Stila's 'Kitten' here ($18); buy CoverGirl's trio '110 Shimmering Sands' here ($5).
All for now! Let me know if you like and want more 'drugstore dupes'! :)
P.S. In case you're interested in the trios - a couple more pictures after the jump.