I actually have both of them sitting on my office display at home...
The biggest differences with the Gemini and MMU versions are as follows:
1] The right hand/arm of the Gemini/MMU is different- where your Gemini astronaut would be holding the manned manuevering 'gun'- the later MMU version only has an open right hand for circling around the MMU thruster throttle.
2] Likewise, the left hands of either kit are going to be noticably different- though some would say not by much...
3] The helmet visors for the MMU vs Gemini astronauts are a different color- the Gemini is a yellow/amber color and the MMU is a smoked "lunar grey".
4] The chest mount components for the Gemini Astronaut kit are conspicuously missing from the MMU kit
5] Essentially the bodies and leg assemblies are identical- the largest differences being that one kit supports the MMU [which includes the "hard mount" backpack followed by the MMU itself] and the other the chest mounted Gemini configuration!
Jot another post if this explaination isn't exacting enough for your needs- I happen to be cheating at work at the moment so some of what I remember isn't nearly as fresh as I'd like it to be!