When it comes to the size question I would mesure the length and width of the sole of boot and email Flow about what size to get.
I've never ridden the M9's but after riding some Fives I am sure my next binding will be Flows. I'll get one of the NXT though because I'm a gear whore.
2011 Smokin Buck Ferton 155
2013 Flow NX2-SE
2013 Flow Hylite boa
Burton Stagger pants/jacket