What 2112 said. Let's not forget that the new pitch functionality of Axe FX 3 will come to FM3 too.
But if you are using a real amp, the reason to upgrade is less.
The Effects of AX8/FX8/Axe FX 2 are great... are the few improvements enough to justify extra expenses?
Also, to get the same amount of footswitches, you will also need the FC-6, and a MIDI to relay adapter. So, the rig will become more expensive, heavier and wider, for the small advantages you'll get... Whether that's worth it, is up to you.