The MA6900 is a great choice if you need power or need to drive difficult loads such as 2 or 4 Ohm speakers.
The MA6500 is slightly less powerfull but has a greater damping factor and results in tighter sound, but not at all less sweet.
I own the MA6400, an older model, less powerful than the 6500 but built with the same philisophy. I upgraded it to the MA6900 for the lack of power I had.
Probably the MA6500 is the best compromise between sweetness (musicality) and being able to controll speakers tightly. If you would go for maximum musicality, consider the 6900. If you need to drive 12" woofers in a reflex system, the 6500 might be more interesting.