I have been investing quit a lot of time to see what is pro, con and the future of MFT, Crop sensor and FF
Obviously each has its niche and I don't see any going away anytime soon.
MFT has 2 main companies that make it, Oly and Pany and they offer very small cameras that can change lenses.
Oly's cameras are directed more at the hobbyist market and they offer great value for the money.
Pany is winking at the hobbyist and pro, their MFT like the GH4 has really shaken the market but mostly the video market, it has a big advantage as a video camera while as a picture camera its ok but has no real advantage over other cameras while APS-C and obviously FF offer better low light performance and shallower DOF.
APS-C is a format that was weakening a bit till Fuji came out with their X Pro 1 and X-T1, its offering a lot of camera in a small package, it aint cheap but not too expensive either, for those who are willing to sacrifice some low light performance then they can move from FF to it and I know of some who did that.
Many camera makers use APS-C as an entry level sensor camera to introduce new clients to their products like Canon's Rebel cameras, Nikon's D3xxx and D5xxx and Sony's a5100, a6000 which offer a lot of camera for very little price.
The crop factor of this format gives sport and wildlife shooters 1.5 (Nikon, Pentax and Sony) and 1.6 (Canon) extra reach from their lenses, thats a huge benefit compared to FF sensor camera.
FF sensor camera is without a doubt the king of the hill, it brings to the table few advantage when the main 2 are excellent low light performance, yes better then any APS-C or obviously MFT and a shallower DOF.
They are big, bulky and heavy and their lenses are big and heavy, even the Sony A7 which has much smaller and lighter body will have to carry big heavy zoom lenses (if Sony will make them) so the advantage of the smaller body is loosing a lot here.
I think each sensor has its place in the market, all formats are capable of making very good pictures but each has pro's and con's
If you see yourself one day moving to FF and can afford it then just jump streight into it just be sure you are ok to lug around a bigger and heavier system, when it will come the time to process the RAW flies you will see the advantages they have especially if the shooting condition had lower light.