I've never used a 1200S, but I have several of the 2x6550 amps from the same era, and I believe that the preamp sections are similar. Generally speaking, they sound great clean, take pedals extremely well, but don't sound that great driven into distortion.
The 2G Model T is a different beast. It has a completely different tone stack, and a mid boost control that is always on. Even at zero, there is some boost. That gives it a different tone. I have a 2G Model T, and I've modded it a little. I removed the blue/white wire that causes the volume to go down as the master volume goes above 8, and modded the mid control so that there's no mid boost when the control is set at zero. I haven't modded the tone stack.
So I'd say that they are rather different amps. The preamp gain stages are similar, but the tone stacks and the splitter/inverter circuits are different, though I don't know how much difference that last makes. In my experience, the T is better for distortion. In addition, it has the master volume, allowing distortion at low levels. The mid boost can make the cleans sound odd, though, and I would expect the 1200S to be better for that.