There are many variables between even the 2 types that you mention, that might determine the choice of an individual family.
- so some Preps end at 11 and some at 13 - that's another choice to be made, depending on the secondaries around you. One that goes to 13 might prep for both 11+ and 13+ and therefore give more chance to wait and see how your child develops and if they are ready to go at 11 or 13.
- some through schools are automatic transfer (unless you're managed out) so can void the stress of exam prep for both parents and kids, but often don't really prepare for alternatives. So these can be good if you absolutely know you want your child to carry on through and know you won't change your mind.....and have a back up plan B in case it turns out your child cannot automatically progress. The seriousness of this depends on how selective the secondary is.....if they take most people, then being managed out is less of a problem, but if the secondary is highly selective and the junior dept isn't, there's a much higher risk of being managed out.
- it's also determined by the intake arrangements at 4+ or 7+. Some junior or Prep schools are selective then and others are not. Therefore, some schools whether Prep or Junior have a wide range of ability and need to manage that and find suitable secondary options for all abilities (more tricky if it's a linked junior and the secondary part is highly selective) whilst others have a narrower range of ability which makes more likely that juniors can transfer directly to secondary - although even with selection at 4 and 7 some kids seem to fail to make the grade.
- another thing is that proper Prep schools are different to Junior Departments. So traditional Preps go to 13 and Prep for a wide range of secondaries.....this in it else's makes the school different because people may leave at 10, 11, 13 and may go to a huge range of places both locally and further away. Schools that go to 13 often have more specialist subject teachers and children will experience less teaching by just form teacher from Yr3 and certainly Yr5, whilst a child in a Junior Dept which goes to 11 and feeds almost everyone into the senior section will have a different feel. Such schools have less variety as everyone is going in the same direction. Schools which only go to 11 often rely on more form tutor teaching and don't use specialists for all subjects - so they may have a sports teacher and a language teacher and a music teacher, but use a form teacher to cover all the humanities and/or English and maths too. So the 13+ Prep perhaps teaches more like a secondary with movement between lessons to different rooms and to different teachers.
So key things to consider ;
- do you definitely want your DC to go to the specific secondary connected to the through private school?
- can they automatically transfer?
- how would you manage if they didn't make the grade and weren't given an automatic place and weren't offered preparation for alternatives?
- how far would you like to keep your options open?
- at what age do the Secondaries in your area start?
- is the ethos at the through school something you want for your older child or is is suitable for your younger child too?
- do you want a traditional prep education or more of a junior education which is often provided by linked junior departments?
I guess the key thing is how set you are on that senior school or whether you want more flexibility.