A top swing front mech is the small, compact version which swings just above the pivot. Useful for some full-sus frames where clearance might be an issue. They should work on most full-suss and hardtail frames though. I think they look nice and neat.
A bottom-swing front mech is the longer one, where the clamp is above the mech. more common on hardtails. Some people think these collect less mud than top-swing mechs.
They're both supposed to swing through the same range.
Rear derailleurs usually come in GS – Med Cage (Grand Sport) and SGS – Long Cage (Super Grand Sport) although Saint Mechs for example come in super-short cage too.
Most full-suss and hardtail frames will work fine with a medium cage. Long cage mechs deal better with chain growth as full-suss frames compress and extend, and more specificlly with cack-handed gear combinations like big-ring to big-sprocket gear selections, which obviously you won't be making anyway. They hang lower though so theoretically are easier to catch on stuff.
If you've got a twin-ring set up at the front then a med cage rear mech will probably be right.
If you're running a small block (like a road cassette) out back such as on a DH bike then you'll want to short-cage mech.
Best bet, pop into your local bike shop and chat it over with them and buy what you decide between you is best, from them. If you do spend a couple of quid more than buying on-line you may well save that in not buying stuff that isn't right in the long-run. And it will definitely be money well spent in the long run when you end up with a great relationship with your LBS who will help you out with all sorts of problems in the future.
