The AD200B and GT200 are different enough that I would love to have both...in fact I seriously explored the idea getting a GT a few years back...actually I was hoping to have Matamp make me a GT with a bigger output section. I am certain they prototyped either a 300 or 400 that carried nearly a $6K priced tag, but I can no longer find anything on it, and I have since stopped collecting.
If build quality is your focus: For equal price, I would go Matamp, however keep in mind you can often find a mint AD200B for around $1,400 US shipped. IMHO the Orange build should provide years of trouble free service unless the amp is abused or incurs a critical failure. Abuse or a component failure could impose critical damage to the circuit board, which would be difficult to repair. The Matamp should last for generations and will be more tolerant of abuse. Component failures are far less likely to result in critical damage as the components are isolated from one another on tag strips.
If tonal character and feel is your Focus: IMHO the Orange has more in common with vintage Fender tone than vintage Matamp. When you run the Orange clean it sounds/feels a lot like big Dual Showman. The Orange preamp is gainy and likes to compress and distort. The output generally is good for about 165 Watts of clean power.
I don't own a GT200 but I do have a GT120 which is very similar in character to vintage GT200s. I have read that Matamp founder, Mat Matias, hated distortion so the original preamp designs were intended to be stay clean until the output was pushed to distortion. As the current Matamp preamp is supposed to be based closely on their vintage designs, I would expect the GT200 preamp to stay clean and uncompressed until pushed hard and the output to be 200 watts or more of clean power. Unlike the GT120 and older GT200s, current GT200s have a master volume and a bit more gain in the preamp, so you can get some drive if you want it. I believe they will tailor the level of drive to your specifications.
GT200 Mk2 adds a mid range tone control. I don't know if they retain the Bandaxall bass and treble controls or swapped the entire Mk1 tone control network out for a Fender tone stack. Reviews suggest the Mk2 has deeper bass and plays a bit louder than the Mk1.
Hopefully you have a bit better idea of the difference between the AD200B and various versions of the GT200. Here's an old thread where you may find even more info.
Video Review of my Matamp GT200
