Amaze is newer than Sensation, and depending on what model Sensation, has a faster CPU. But it was recently announced that the Sensation will get Android 4.0.x (ICS) sooner than the Amaze. That could be good or bad, depending on your perspective.
Amaze has higher res display that Samsung's SGS2, and I think the colors are better.
At the end of the day, both the Amaze and SGS2 are superb phones, many reviewers and editors have said that a buyer shouldn't be unhappy with one or the other.
I suggest you read reviews and comparisons on places like Gizmodo, Engadget, PhoneDog, Cnet, etc. and then sleep on it.