My DS who took GL (in AGSB) and CEM (in Sale) scored 30(ish) marks higher in GL than CEM. Difference may not (fully) point at hardness, as he is good in Maths (not so good at comprehension skills). Level of difficulty (according to him) CEM > MGS > GL
Just to bring it to the notice of people who are preparing for 2017 entry. For CEM, you may need to aim at a score above 350 in the past papers.
I think it is a waste of time quantatively comparing the two. Better to compare raw scores and standardised scores between two CEM tests administered under the format using the same age standardisation and weightings.
Good mathematicians doing CEM in one area with a higher emphasis on vocab will score lower other things equal.
MGS tests qualitative aspects such as ability to infer and show approach. e.g CEM might say
Q1 light is nearest in meaning to
a) dark ) transparent c) camper d) colour e) bright
whereas MGS might ask you to describe the effect turning a floodlight on.
Crude example, but highlights the lack of comparison. Tons of overlap, but you must compare like with like