This depends a lot on the feel you are looking for. The Five standard gets a one piece highback and a nylon base. It is noticably more flexy than the Five SE (or the M9) which gets an aluminum heelcup from the mod-back. The top panel on the mod back is very flexible but on the whole it is still more of a power binding than the Five due to the aluminum usage. The Five SE and the M9 are very similar with the differences being the flex in the top panel and the design of the surf strap.
So if I understand correctly:
The FIVE SE and the M9 are as an overall feeling quiet similar?
Any reason to choose the one over the other?
I had 2010 or 2011 M9's in Large but my boot (32 Lashed FT in Size 43EU) was little too wide for those. Very, very snug fit. I now passed those to the misses board as she also has wide boots (32 Prion FT in Size 38EU) but at least the width is ok. (Needed some tuning to get her boots fitting perfectly, by advancing the highback, but I think that worked out)
Better to diss out some more bucks to get the NXT with the NASTY system?
I'm spending half of my boarding-time teaching youngsters, the rest is all-mountain trickery :-)