It’s very similar if you read this thread: the GC vs MML was also an easy one as the MML served far more significant places.
BIB - Not true though, both served Leicester, Nottingham, Chesterfield, Sheffield, Manchester.
The MML served Derby - which the GCR didn't.
South of Leicester - the MML served Market Harborough, Kettering and Wellingborough - all of which were much smaller in the 1960s than today. We'll assume for these purposes that the Bedford - London line would have remained open in the same way Aylesbury - London did.
The GCR of course served Lutterworth and Rugby, the latter probably being more significant than Kettering or Wellingborough in the 1960s.
Going north of Sheffield the MML headed to Leeds - but was always a secondary route when compared to the ECML.
Really not comparable with the GW mainline which from Exeter then served the south Devon coast which is, compared to the SR route over Dartmoor, well populated.