I have anAqua Blue 3DS. I have this one because I really like the blue color... (It's somewhat easy to see if you know me well enough.) And Aqua Blue looks like a kind of blue and green, and I like both colors, so it's a perfect one for me and it's just awesome...
I can't stand having two different colours on the 3DS. I don't think it looks good and annoys me quite a lot. The Cosmo Black 3DS; however, looks too boring.
I have Cosmo Black - though I wanted Aqua Blue, I didn't want to get the same 3DS as my older brother (who seems to get his way with these things all the time :what