One is just a pulley on a bolt. And the other goes on a more complex bracket. I am not sure on the consensus on replacing just the pulley or the whole assembly.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)