Hey Pogre,
AMTs and ribbon tweeters are not the same at all, at least in design. AMTs use a folded membrane, and it collapses and expands the folds to squeeze air in and out of these folds. It does so at a frequency determined by the voltage signal. Ribbon tweeters oscillates a light film back and forth as a whole. AMT tweeters tend to be more robust, but they can range in quality from good to blah. I have heard very good AMTs and also very good ribbon tweeters.