For posterity's sake, here is the outcome of my home shootout: The POD XT Live is going back to Guitar Center today. To make this as fair as I could, I used each MFX box exclusively for a couple of days, going through the presets and then editing patches to get a couple of good clean sounds with and without effects and a couple of good distortion sounds, with and without effects.
The last day was spent A/B testing my "best of" homegrown patches on each unit. The same two guitars were used throughout (a MIM Strat and a Carvin Bolt with a humbucker in the bridge), and I plugged into the effects return of my Tech 21 Trademark 10 amp. Not the most ideal amp to use, but I found that it provided the least coloration. When it got late in the evening and the wife started complaining, I switched to a pair of Grado SR-125 headphones. The bottom line is that I tried to keep my testing as fair and even as I could.
Below is quick list of pros and cons for each unit:
Line 6 POD XT Live
Pros:
Outstanding software interface (works on Macs too!), nobody comes close to Line 6 here
USB interface for quick-n-easy PC recording and patch editing
Very good high gain sounds
HUGE community support for troubleshooting, patches, etc.
Excellent online patch trading/downloading system (customtone.com) with tight integration into the editing software
Line 6 updates their software/firmware regularly and it's a piece of cake to upgrade/update the unit
Cons:
Lower gain sounds sound more artificial than GT-8
Effects lack depth and realism when compared to GT-8
Unit does not respond as well to pick dynamics
Unit does not respond as well to volume rolloff on guitar
Several amp sims have a distinctive "Line 6 voice" about them that I didn't care for
Boss GT-8
Pros:
Lower gain and clean patches sound excellent and realistic
Effects are noticeably cleaner and have more depth than XTL
Dual processors for running 2 amp sims simultaneously
Most patches sound like distinct models rather than variations on a single patch (see "Line 6 voice" above)
Good response to picking dynamics and volume rolloff - not quite tube-like, but significantly better than XTL
Overall, feels more like playing a real amp than the XTL does
Cons:
No USB connectivity for patch editing or recording. It does have SPDIF out for recording and MIDI connectivity for patch editing, but it's not as easy to use as USB
Patch editing software is unavailable for Mac (but there is a hack for the GT-PRO editor that works reasonably well)
Smaller user community for troubleshooting, patch trading, etc.
Steeper learning curve than the XTL, there is a dizzying array of things to tweak
If you've read other comparisons between these two MFX boxes (there is a great writeup here by Kewlpack and several user reviews over at bossgtcentral.com), you'll see that my impressions pretty much line up with other people's thoughts. The POD XTL is easier to work with, but to me the GT-8 just plain sounded better and had a more realistic feel to it. Ultimately, that's what it's all about - getting the right sound and the right feel. Editing the patches on the GT-8 can be a challenging and frustrating experience, especially after using the user-friendly Line 6 Edit, but the results are absolutely worth it.
If Boss put a USB interface on the GT-8 and supplied software comparable to Line 6's offerings, the GT-8 would be nearly unbeatable