the GT8 is a much 'deeper' box, which allows lots of control over different parameters and effects, 2 preamps selectable per preset, and effects that the PODxt simply doesn't have.
The PodXT sounds better right out of the box.
The GT... you need to spend some time with it to get the most out of it...when you do, I think it sounds better...I had both, and kept the GT (I have a GT-Pro).
And just to ad to your delema, have you looked at the VOX tone lab SE? It has two built in expresion pedals, cab choices, amp styles, pedal selections, modulation type, reverb type plus their valve reactor technology that makes those VTX amps so desireable
This is only 3rd party info, and personal research - I don't own either: It really seems the GT-8 is the box of choice. I'm thinkin GT-8, Crate Powerblock and ?? Cab.