Two very different drugs. I've tried the 2nd synth sample of 6apb and it absolutely knocked my bollocks off. Mind you, I did fuck up my measurements and took way too much, but it was very MDMA, pushy, full OEV experience. Very very enjoyable. Felt like I'd had a very heavy experience when it was over.
aMT is much more subtle on all fronts, whilst still managing to hit all the right spots. The euphoria is consistent and not overpowering; at no point do you feel as if you are not in complete control. Of course, you're fucked, but not 'out of this world'. The visuals are also very different. On 6apb, I actually had tangible objects (bright blue orbs) in my field of vision that I could move around and play with. aMT is much more trippy and the visuals play around with the environment that's already there.
I say try and both and pick your favorite. I think they both have their strong points depending on your situation.
I would caution though that the new 6apb pellets do not appear to be anywhere near as strong as the 2nd synth. I think (for many reasons which I won't go into publicaly) the lab have had to scale back the amount of the 'good stuff' in the pellets. Not by much, but certainly to achieve what I achieved with the 2nd synth, you're gonna have to spend about £40-£50, which makes aMT by the far the best cost wise.
Enjoy!!