As for differences with the different pods etc...
Pod 2 is the main thing youd want to look at, its the most supported by line6 really, lots of models heads/cabs and effects.
Guitarport, which ive owned, mainly for direct computer recording, in my opinion its not so good.
Toneport, again computer recording, a little better then a guitarport and has more options like xlr input and output to connect into amps or pa systems.
Bass pod, well, for bass not tried it.
Pod xt pro, the rackmount version of the pod2, same setup/sounds as the pod2 but has more outputs for live performance.
I have used and do use guitarport and toneport, theyre good for practice and recording quick ideas down, you can upgrade all of the pods etc with model packs, you can get loads of sounds but recording to computer you dont get the same punch that you get off a real life large speaker/cab setup, maybe if you used a pod2 through some form of poweramp to cab setup youd get the punch then, but ive never tried it.
For home recording theyre great products, but despite what they say in their adverts you wont get the studio quality sound.