We have just moved from Cheshire, where most of the schools were 11-18 with inclusive 6th forms ( obviously) and most kids went through the system on one site or went on to the local FE College for other subjects/vocational courses etc - it was a system I thought I prefered, although my child's HS wasnt that great, the 6th form had a very good reputation, and I thought it was aspirational for the younger ones to have a 6th form attached. But if the child has had a bad time at HS for whatever reason, there is no opportunity for them to 'escape' and try something else.
We have now moved to a county with 11-16, and they are big schools with good reputations, with good 6th form colleges with excellent reputations, so there is another move for your child to contend with. For my child, the move has been brilliant, she has really grown up alot at 6th form college, they are treated like adults, and it feels like a real move away from 'school' - different site, different ethos.
I am still weighing up the two systems really - for us a family move meant we had to move at this time, and I can see what a difference this change in system meant for my daughter, she was ready to get away from a familiar school environment and have a new challenge.
In general however, I do question large state comps really - my son has just entered year 7, and although has fitted in well, I think that a middle school type system would have been preferable for him, and that the schools should come up with systems to keep year 7,8,9, operating separately from the much older ones - this is just my opinion based on our experience, - children have different needs and respond to different systems.