I have
a GT-Pro and a POD xt. IMO the PODxt comes nowhere close to the GT-Pro for sound quality, tone and realism. I only got
my GT-Pro about 2 months ago but already I realise it is a completely different animal to the xt. From what I have read, the GT-8 and the GT-Pro have almost identical tones - the GT-Pro is just more flexible when it comes to connections and routing into rigs, cool for reamping too.
A lot of guys say the PODxt is the best out there, thats mostly because they have never tried anything else or have only tried inferior products (no insult intended).
At the end of the day it depends on your musical tastes, the PODxt can probably produce the best tones in the world for certain types of music, just not the tones I want.
The PODxt has it's good points, the software is very user friendly (as is the GT-Pro) and Line6 have excellent continuous development - loads of driver updates and changes on the website. I tried my best to like the xt because the software interface is excellent and it's small, light and portable. I just couldnt convince my ear so now my PODxt is on its way to Ebay.
Try the GT-8 and the PODxt side by side before you buy. Also try the Digitech GNX4, that's got some solid tones too.