I have to say I love my MT RTA, and it is very well liked around here. Check out the thread in this section. I don't have the Griffin but would like to try one, though I don't need the JFC. My guess is the MT has superior wicking with its huge
juice channels, particularly for thick juices. It has the best wicking of any RTA I've ever used. Griffin might be the better choice for thinner juices because of the JFC. And of course you get the velocity posts which, along with the bigger deck (same size as MT), will make it nice to build on.
For max VG, MT has an advantage IMO. For features and for thinner juice, Griffin.