I've had them both and they are very different from each other, if your goal is to bash and some time on the track then get the GR3 (hands down!).. if your going to race and do very little bashing, then go with the G3.0 & G3.3 suspension (hands down!)
Motor would choices would be the following
1515 3d s
1515 1.5y s
I might / maybe reconsider your (2) 7.4 idea, I'm running on (2) 3s 5400 = 6s and it fricken awesome!, 4s is ok and you'll be fine (lows / mid 30 mph) It a safe route and you wont really be going fast enough to brake anything (that's a positive).. I've never run a center diff on a MT so good luck with that one, I here good a bad things, so I’ll be looking forward to your future updates.
Both trucks look flippen awesome when constructed . Here’s my version of the GorillaMaxx GR3 and G3.0.
And here are the Videos on each truck.
http://www.youtube.com/watch?v=8mLH3caydes
http://www.youtube.com/watch?v=3Svd8ZDnS6Q
Many other team members also have bitchen rides as well look at the following builds to help in your selection;
Jordan
Suicideneil
Chase023
Ssspconcepts
George16
Feistyacorn
To just name a few.
Best of luck in upcoming build.