Wood, I think there are elements of both.
From a teacher's POV, there are issues around 'absolute ability' - access to higher level work, pace of progress, test ceilings, ability to manage cross-phase transitions (e.g. primary to secondary), all the discussions around acceleration vs depth, curriculum content vs non-curriculum enrichment etc.
From both a teacher and a parent point of view, there are also issues around relative ability - the extent to which a child is an outlier in a specific environment and is therefore perceived as 'different'. The specific way this impacts on a child will depend, to a large extent, on their own nature and other skills - a range of hobbies, or none; socially skilled or socially awkward. However, there are some common theme around 'how worthwhile / feasible it is to provide for a group of near peers', rather than 'provision for 1 very different individual', and simply around the experience and knowledge teachers and other staff have of dealing with a child of this level of 'difference'.
It has its parallel at the other end of the ability spectrum When woking weith a child with significant SEN in school, it is both the severity of the SEN, and the presence or absence of children with SEN of a similar level or type, that affects how easy a school may find it to meet that child's needs.
However, an over-riding factor in all cases will be the general 'philosophy' of the school, and the willingness of staff to 'go the extra mile' and to see ability as 'exciting', rather than 'a nuisance'. Tjhis is not a function of the type of school, and as others have said, can as often - sometimes more often - be found in schools with VERY mixed intakes than 'n 'naice neat MC schools'.