My DCs attend a prep that goes to 13 but has a sizeable proportion (mainly girls) leaving after 11. Years 7/8 tend to be boy heavy and focused on those going to schools which start at 13 (either boarding schools, or day schools in London - KIngs, St Pauls, Westminster, Dulwich etc). Most of the boys do a pre test in year 6/7 for their chosen secondary and finishing the 11+ curriculum in year 5, doing VR and NVR and revising it during the first half of yr6, with many practice papers (yawn), means they are well prepared to work in an exam situation under pressure (although there are some pre tests you cannot prepare for eg. computer driven tests).
For me the reason we stayed for years 7/8 was driven by our choice of secondary which started at 13. DC1 flourished during years 7/8 - getting to the top of a school and taking on responsibility, being in a class of 12 so the teaching can be really personalised and frankly, growing up and making mistakes in a smaller more nurturing environment were other plus points. This is for my DC - others may be different with DC that are ready to leave at the end of year 6, or need a change.
At our school the 11+ prep is just as important as the 13+ prep, although the nature of the exams is different - 11+ is competitive (eg. 500 sitting for 100 places), whereas 13+ CE tends to be qualifying (once you've got a conditional place after a pre test, you need to pass CE at the required mark - I think failure is very rare). Non CE 13+ entrance is obviously competitive, like 11+, so it depends on your chosen secondary.
At the end of the day your prep school should guide you through the process and recommend schools that will be a good fit for your DC. At ours this process starts in year 5. Does your chosen prep have a good track record of getting DC into your chosen secondary school list?
Good luck. Sorry so long!