Here's my opinion :
GT2 : Nice, thick sounds. Very configurable, you can get most sounds you can think of. Only downside is the lack of three button switches, quite a pain if you like having a few tones ready. It does not work very well with some amps, have to tweak and tweak.
Tri-AC : The obvious plus pts are the mid-control and the three switches. Overdrive not as smooth sounding as what the GT2 can achieve. Pretty grainy type of overdrive, not the kind I like. The tweed channel is almost useless with higher gain, even when I tried overdriving it with my sd-1. Just plain bad, I dunno why. The british channel can get a nice drive sound. It sounds better with amps to be, maybe due to the cabinet simulator.
If you like clean, the GT2 is slightly better.
Overall I like GT2 better, but now I sold all my pedals haha.
Pier.