The biggest differences that I noticed are that with the PodXT is that you can put all the effects(overdrives, modulation, delay, wah. volume pedal etc) in either a pre or a post configuration. And there are lot's more types of effects to choose from. Analog delays, stereo delays, digital delays, more types of compressors, overdrive/distortion stomp box modelers and on and on. It seems to have the Line 6 modeler pedals all built into it.
You have different mics to choose from. More amp and cab models. You can choose if it's going into a pa/direct, infront of an amp or in front of a power amp/4x12 cab combination via the nice display on the PodXT. It's not just turning the cab simulator on or off like the older Pods. Another thing that's cool about it is that you can totally bypass the amp and cab modeling and just use the effects from it. That's something that really turned me away from the other Pods. Also, I pickup up the FBV Short board for my PodXT. At first I was kind of dissappointed that you couldn't use the older floorboard with it but the new floorboards are much better. The short board that I got was only around $20 more then the old floorboard and it is also much improved. It has the nice well lit up display that shows what program you are on and larger, more visible lights above each footswitch. It also has an effect loop on/off switch, which I'm sure is for the Vetta and the Duoverd(?) amps but it also works as a boost switch for the PodXT.
I would suggest that if you have a local Line 6 dealer, go in and ask if you can look through the manual for the XT(If they have them in). That's what I did and I bought it. Also maybe sit down with the PodXT and play around with it for a while before you buy it. It took me a while to find a sound I liked but once I did I was blown away.
I've had all the other Pods, Pod 1.0 Pod 2.0 and the Pod Pro, and the PodXT basically leaves all other versions of the Pod in the dust.