I've only used the demo version of GR, but the cab sims were much better and WAY more versatile than the PODxt. I didn't like any of the amp models though. The best way I found to use it was to run your preamp into your soundcard and then set it up so the signal goes through GR and then to your recording software. How you set your preamp will depend on the cab/cabs/mic/placement you use, so it's best to set you base tone (with cab sims) up before recording. If you set up your preamp and record it dry with no sims, and then add the sims afterwards, you're stuck with that one tone to work with. You might like the characteristics of a certain cab, but it might sound like crap with the settings you recorded with. That's why I treat it like a real live rig first, and then record.

I've never used the Palmer, but have heard some really great things about it. It's best feature is it's ability to let you run your tube amps into it with no cabinet, so you can have fully cranked tube tone through headphones if you wanted.