Personally, I like the -21FP.... Except that it's purple, but that's another issue.
Transformation: Is REALLY tough the first time. and I think it will be tough the second time too. I've transformed mine into each mode once so far and now she's back in the box for the time being until I get my display case built.
I say, if you like the M+ line then get one. It looks great in all modes I think (especially fighter). Like Abombz!! said though, try to find a good price on it. I paid full retail, and I'm ok with that, but if you can find a better deal then by all means.