At the risk of stating the obvious, let me boil down for you the single biggest difference:
Fantom G:
built in keyboard
MV:
no built in keyboard!
In my case, I already had and loved my Yamaha CP33, so I went the MV route, and in the process I did get 3 things that the Fantom does not have:
(1) fader automation
(2) more portability
(3) better (IMO) workflow. The interface on the Fantom is more modern (I love it, by the way), but the simpler unixy-look of the MV is also quite appealing when you get used to that look too. But the larger pads and more dedicated sequencer-oriented buttons give the MV a workflow edge.
That being said, if you don't put alot of weight in fader automation (warning: once you get used to it, good luck going away from it!) and especially if you need a keyboard, the Fantom might be the better choice for you.