Well I've used both now and they both have their own merits
MDT is great for new network windows and windows 8 with various devices and various amounts to image and all software deplorable with Msi software deployment.
It can handle windows xp image capture after sysprep which is great, and it can handle application deployment direct from mdt or build them into the deployment image.
FOG is great for various makes and models with various os's and non Msi type software deployment, you make the images complete with all the software installed and then capture image and deploy.
You can wake the stations up and image overnight from the fog console, this is something that can't be done with mdt but can probably be addressed with sccm.
Fog also performs and inventory of each station in the form of make of motherboard and memory and all sorts.
Again mdt was never designed in this way.
This is how I see them being used
Take a small village school with 10 or 20 computers, they need software the same on all of them you can take a laptop in with fog on it and and create youre first image then capture and deploy to the others
You couldn't really take a windows server in on a laptop setup mdt and deploy, well you could but it's normally a domain and you deploy windows 7 with apps deployed with gpo's.
So not really viable.
Fog though doesn't care about a domain it's just and imaging piece of free software.
So in a nutshell both great and totally way in advance of the retarded rm connect build and deployment.