I think to accuratley test them you would have to set the mental ray scene up better. Just setting things up to the right dosn't mean it is the right setting. If the renders treat certain lighting setups differently you should compensate.

Im also not sure what it is you are trying to test? Becase if you are testing speeds i would think that the end result would have to be identical. At the moment the pictures are too different to be accurate. Showing two completley different pictures and saying one is superior because it is faster is not accurate. If you could show me two Identical Pictures and say that one is done with Mental Ray and the other is done with Maxwell then maybe the test will have a point.
At the moment it just looks like you are trying to make Maxwell look much better than Mental Ray.

Just my little opinion