I have tried both and I have a TRI AC now.
Actually, the TRI AC's amp sim sounds better than the GT2, which sounds a little flat.
Main difference between the TRI AC and GT2 would be the voicing. The GT2 is more modern voiced while the TRI AC is vintage. The GT2 is a monster on gain, IMO, no need for boost pedals to achieve crushing death metal gain, and of course, it can do clean, jazz, blues and so on... However, its forte is heavy gain or modern tones.
The TRI AC is vintagey and has little gain compared to the GT2... you need to have a booster pedal to get anywhere close to a tight metal tone. However, it does blues rock and jazz VERY VERY well. Its more responsive/tube-like than the GT2 IMO... and you can get metal tones out of it with a TS9.
In a Live Application, both can plug straight to the PA or whatever amp is there, so you don't need to bring an amp. The TRI AC wins hands down for versatility: you got 3 sounds to play with while the GT2 only has 1 (on/off).
I decided to get the TRI AC because I wanted 3 tones and no amp. I lug around my pedalboard now, and I have the TRI AC set to clean, crunch and heavy. Throw in a ts9 for breakup, heavy blues and metal.
But if you want hi gain stuff, go for the GT2. Btw, the GT2 to the TRI AC to speakers sound ridiculously heavy!!!
Check the sound samples on www.tech21nyc.com
to get a better idea of this vintage/modern voicings i described... their sound samples are accurate.