I'm with Chas; C3's beat Aliens, especially in the Gunks, where head width can make all the difference between getting something in and having nothing. The only Alien that is still useful is the green one, because it fills an unfortunate gap in the C3 sizing.
(Full disclosure: I still have Aliens on my rack besides the green one, 'cause I own them and I haven't sprung for a double set of C3's. But having both makes the superiority of C3's evident.)
(More disclosure: in the two sizes above a yellow C3, the best cams are the blue and yellow Totems---not the Totem Alien copies, the real Totem cams. Actually, I think all the Totem cams are the best cams in their sizes.)