What is the difference between the two? I am looking for something that will work on my 2003 Giant Iguana MTB. Any help is appreciated.
Top swing will get faster shifts w/ a bit less effort compared to a bottom swing, but not really much difference functionality wise of top vs bottom (some FS designs can only use one or the other, depending on how the rear triangle is mounted). The bottom swing has a longer parallelagram (old school), so mounts higher. Either swing will work for you.
You need to make sure you get the right PULL -top (TP) or bottom (BP). Look at the existing derailleur. If the cable pulls from the top, it's a top pull, if the cable routes under the BB shell and pulls from the bottom... you got it.
ps check out some derailleurs on the shimano site to see what I'm talking about