I've had my G-Dec 3 Thirty for a few months now and I love it. I can't comment on the others you mentioned because I've never tried them. I have limited patience when researching/trying out stuff and tend to be an impulse buyer.

I have no regrets for jumping at the G-Dec, it's just what I wanted. I like nothing better than playing along to the huge number of backing tracks for hours. The sound is excellent both for the guitar tones and the backing tracks. You can alter the speed/pitch of the backing tracks but to be honest the sound quality suffers. I don't see any benefit considering the number of tracks available in different keys and tempos. Maybe a more demanding player might I don't know. There are so many options for emulating other amps and all settings can be tweaked to fine tune it for your own preferences.

The USB connection is good in that you can tweak all the amp's settings on the computer (FUSE has had it's teething problems but it's now stable thanks to V2.01) rather than hunching over the amp all the time but altering stuff on the amp itself is a breeze - just make sure you put the amp on a stand or something to save your back!

The SD card gives you the ability to save all your preferences and move them on to another G-Dec or you can store stuff on the amp itself or a PC, it's your choice.

I now have a damn good sounding amp with loads of options that is perfect for me to play along to. I need look no further for a long time to come :D