I've skied neither, but it is my understanding that the M-Pros get less turny / more inclined to do slightly longer turns as the they grow wider - with the 90 still being more inclined to do smaller turns and being very forgiving to ski.
The 90 is supposedly still more of a frontside carver than a big line slayer (compared to the two wider m pros, especially the widest), so it could be that the jump to a more advanced construction and slightly wider ski could be the best of worlds approach you are looking for.
some tests:
84,
90,
90, there are also a fair few videos on youtube for either.