I think AGSB is meant to be more academic than MGS isn't it? I think the AGSB entrance exam is certainly meant to be tougher?
I think maybe revisit the stats for both schools .
All stats A* and A*/A at gcse and A levels are freely available in the pubic domain( for 2013 and previous years) and past papers from MGS and GL can also be compared.
With respect I think you may change your opinion?
the catchment for MGS is huge they attract the best and brightest form places like the Golden Triangle (Wilmslow, Prestbury and Alderley Edge) as well as Didsbury , Bramhall, Worsley,Knutsford,Lymm ect who do not have access to the Trafford Grammars some come from upto 30 miles out!
The strangest thing is a lot of pupils from Altrincham( particularly from Hale and Bowdon) also go there when they are actually in catchment for Trafford Grammars!
population- wise Trafford is one of the smallest LEA in GM- and has 7 Grammars in a geographically small area -all catchment dependent , no proper super-selectives
So why do you think that any Trafford Grammars are more selective than MGS a top tier Indie School?