There's probably a variety of answers depending on the surface (artificial turf, grass, muddy conditions, etc.) but when I kicked in high school (they didn't have a soccer program) I used my regular everyday soccer boots. I've seen some have a longer cleat on their plant foot to make sure they didn't slip and then just about anything on their contact foot that was comfortable.
Many kickers do this mainly because football cleats aren't meant for kicking the ball. I'v seen lots of players with their football spike on one foot, and their Adidas copa on the other.
that is what i see most of the time as well... in college, the kickers tend to wear copa's on their kicking foot...i dont watch a lot of nfl games so i wouldnt know what the pros wear...
i'm the kicker at my hs. i wear tight copas. its basicaly the same as with soccer. you want a clean striking surface. and copas are a nice clean boot. i dont like the two different shoe thing. personally it looks stupid. just wear the same 2 shoes. and if its wet, wear sg's.