I would go against the majority here, I went to a 3-18 school and I loved it. There was a clear divide between pre-school (3-4), primary school (5-10) and the senior school (11-18) which made for opportunities to transition and start afresh. Yes, at the end of year 6 I wanted a change of scene, but that was provided by the move to the senior school. It also meant that there was an ability to interact with people of all ages at school, and lots of babysitting jobs 
Of course, if you are going to pick a school that goes right through you have to make sure that
a) the school meets your standards at all stages
b) the school satisfactorily deals with children who perhaps they would not pick if they met them age 11.
c) there is a way for you to move schools if it all goes downhill.
To clarify b) I mean that if your child is selected because they seem bright and bubbly at 4, but then turn out to be a remarkably average child, or are diagnosed with SEN, or suffer a life changing injury, that the school will not just ditch them.
In my experience, a "right through" education was the correct choice and I had a brilliant time and made friends for life. For your child and your family, it might not be the right choice, or it might not be the right school. Pick carefully but remember it's not the end of the world, believe it or not you can use a 4-18 school's prep and then transition out later, even if this is not encouraged by the school.
Also, I just re-read your post and noticed that the preps are much closer. This might be the easier option for you then. Or, is it possible for you to use a local prep until, say, 8+ and then move to one of the 4-18 schools? It would avoid the big hassle of the 11+, but it might bring its own challenges.