I've got the opportunity of get either of these units for around the same price. How do you guys think the two units compare?

I've not played either in person, since they are both out of production, although I have played a Boss GT10, which was pretty good and I also have Line 6's Pod Farm VST, which I think has some of the same sounds as the Pod XT.
Pod XT. The GT-8 has fizzy gain tones and a terrible wah. The XT sounds decent, has USB audio recording, a very good preset editor, and Pod Farm.

The GT-8 needs a midi connection for preset editing and the editor software is ok if you can get it to work, but good luck on that. BTW, the GT-8 editor is just a 3rd party homebrew tool. The only advantage to the GT-8 is that it is good for 4CM hookup.
Pod XT Live is the much better choice (unless you're looking for effects only).

However, I'd recommend you look into the RP1000. Digitech raised the bar quite high with that modeler. Plus you can find them on Ebay cheap.
1979 Gibson Les Paul Silverburst
James Tyler Variax JTV89
Schecter C1 Classic
Ibanez RG520QS
Greg Bennett Torino TR4

Mesa Boogie Dual Rectifier
GSP1101 & Pod X3 Pro
Peavey 5150 & JSX
Bugera 6262 & 333XL
Carvin V3
Spider Valve HD100