For use with an amp, the Boss GT-8 is pretty much outright superior to the Pod XT Live if you ask me.
The Pod has a bunch of synth effects, and some are cool, but they aren't really adjustable and most of them are useless, just like with the amp models. They're basically just random arrangements of noises and harmony settings that you can't change. It also just has a pitch shifter effect that does not do intelligent harmony.
The GT-8 has just a few synth effects, but they're extremely adjustable, and you can run the intelligent harmonizer with up to two voices, a synth effect, reverb, delay, chorus, OD/distortion, and two amp models all at the same time, and then arrange everything in any order you like, so you can basically make your own synth effects. The feedback effect is also pretty cool for making weird sounds.
I hate the way the Pod's effects groups are layed out. It's nice that it has those extra footswitches for them, but since you can't adjust what does what and you are limited to one from each category, it's pretty useless to me in comparison. Even though the GT-8 doesn't have extra footswitches just for turning effects on and off, you can still put it in manual mode, and then it can turn just about everything on and off with a footswitch, so that's probably better for most people, anyways.
The GT-8 has a loop, channel switching capability, and is just way more adjustable, which I think makes it a lot better suited to use with an amp. I think the amp modelling is a bit better, too. They don't sound terribly different, but once again, the GT-8 is far more adjustable and it's easier to get what you want out of it between the two amp channels at once thing and the much more parametric-like EQ. You can also do some cool stuff with the stereo two channel setup and the thing where it changes between the channels depending on how hard you pick.
It also has the sound hold feature where you can press the CTL footswitch to make it hold whatever you are playing, and then play something over it. I haven't really messed with that, but I'm sure you could do all sorts of strange and inventive things with that.
The only possible drawback to the GT-8 is that it does not have USB. Just SPDIF. Although I don't really record direct, I actually prefer that, because then I don't have to go changing drivers around with some of my programs, but I'd imagine lots of people have no SPDIF connection.
If you don't want the synth stuff, though, you might consider the Kurzweil Mangler and a MIDI controller.
The GT-8 is actually a very good unit, but I dunno how the PodXT Live is. I've actually never used a Pod in my life so....
