If you are paying $1000+ anyway, it would be wiser to buy an iMac. The only advantage of the Mac mini is that you can choose your monitor.
It is very easy to add memory in the iMac, even you can do it. It comes with detailed instructions on how to proceed. Note: this is not the case with the Mac mini.
According to MacUser UK "Maya 6... Mental Ray is a third-party renderer, which is used in many other high-end 3D systems, including SoftImage XSI and 3D Studio Max".
So the modelers may be different, but there doesn't seem to be any difference between the two products as far as rendering goes.