my uncle has the 500, it's a pretty cool all in one. i haven't used the recording features but, pod has been around a while, so they've probably worked most bugs out a while ago. as far as differences in the audio interface, besides different feature sets like adat or more channels, ect, there are differences. but if your just talking sound quality/performance of a basic 2ch interface, one is probably as good as the other, and should get the job done.

a seperate interface would not have modeling, so you would send a clean signal into the computer, and have the modeling done by an amp sim program like guitar rig. if your computer is decent you shouldn't have bad latency (a delay between when you play a note on guitar, and hear it back thru headphones/speakers), and still be able to run a couple instances of the pluggin. using the pod may help in the latency/processing area, since you wouldn't have to have the computer doing the amp sim, just recording a track. You'd really have to compare them for yourself to see what sounds you like better.

If you would consider a high-end audio interface with a great preamp and instrument inputs with high quality converters, it would certainly be better than the Line6. How the instrument input and converter takes care of the audio signal can change drasticly how your track will sound. Plus it would give you the possibilty to record other instruments (voc, bass...) and of course use 1 or 2 mics in front of a real guitar amp (which is my first choice). But, if a 90$ audio interface is what you had in mind, I'd definitly go with the Line 6. I bet the analog circuitry and digital converter will be better.

Ok thanks I'm ordering the line 6 soon.