I'd like to add my two penneth worth to this discussion as our personal experiences have helped us see the pros and cons of both systems.
Background: our eldest DC is very bright, quirky and an anxious child by nature. She's excelled academically, but is slower to thrive socially. Our youngest is bright, but not as academic as DC1 and is one of those 'will thrive anywhere' children.
Where we used to live, the only comprehensive school that our DCs could get into was huge, had 'inadequate' Ofsted reports and very poor results. We were 20mins drive from the nearest grammar school, but their catchment area extended in the opposite direction, meaning our DCs could score the highest mark in the test, but still not get in based on catchment criteria. We could not afford a house within catchment.
We moved. We now live next door to a super-selective grammar and a comprehensive school.
The comprehensive school utilises a fair banding assessment that is taken before pupils know their 11+ results to set what proportion of children they will let in of each 'band'. Children are let into each band based on distance, but as some of the 'top band' end up going to the grammar, the reality is that you can get in from further away if you are top band. This seems unfair to the pupils in lower bands, as they can live a mere mile away and not get a place, whereas a top band child may live 20 miles away and get a place. It does keep the school truly mixed ability though.
The grammar is super selective and therefore children are tutored extensively and travel from long distances to take the exam. This has meant that very bright pupils, who live right near the grammar, have not secured a place as they've just missed out when competing with the very brightest/most heavily tutored from a huge geographic area.
The local area is crazy at pickup/drop off as many parents drive their children long distances - I'm sure the environmental footprint of the school is large.
DC1 has a place at the super-selective and, as we are starting with transitions, we are realising that, even though we live in walking distance, DC1 may have no local friends.
I think the most sensible option (if we are keeping grammar schools at all) is to have some sort of halfway system where rankings and catchment are taken into account.
Ultimately, the system will always be unfair when people can move closer to better schools (bumping up house prices), pay for tutors/private education etc.