Originally Posted by MattP.

Top swing and bottom swing refers where the derailleur is anchored the seat tube. They are pretty interchangable unless you have a waterbottle braze on which would limit you to either top swing or bottom swing.

What you should be more worried about it whether you front derailleur is "top pull" or "bottom pull" because those are not interchangable. If you cable comes via the downtube to the bottom bracekt shell, it is a bottom pull. If it comes via the top tube it is a top pull.

As far as what rear derailleur to run, it all depends on what gearing you are running. A common gearing would be 22-32-42 in the front (referring to number of teeth) and an 11-32 (or 34) in the rear. If you are running that, go with a long cage. The derailleur needs to be long enough so it can take up the slack from the chain.

An interesting article: http://www.sheldonbrown.com/derailleur.html