Mac Mini versus iMac for first time user
i the Mini you can still add more RAM i have seen if you can on the iMAC but i doubt it with the new slimmer design.
if you are talking about just using the iMovie application i think you will be fine with the Mini assuming you have the keyboard, mouse and monitor already.
For light iMovie use, I seriously doubt that you will miss the GPU.
Macworld posted some benchmarks that do show the difference between integrated Intel HD Graphics 4000 and the Nvidia’s GeForce GT 650M earlier this summer in their comparison of 13" and 15" MacBook Pros (see Portal 2 benchmark): http://goo.gl/E3OKy