I have 2 sons, one at a boys prep and the other at a co-ed private primary. The schools have different styles eg for the prep schools everything is competitive, sports, music, getting a part in the school play etc whereas the smaller co-ed is very inclusive. So anyone can join the choir, and everyone gets to be in a play, be on a sports team etc. One of my sons thrives in a competitive environment, the other one doesn't (small for his age, young in his year, not particularly sporty).
Academically the standards are different too. One school does timetables (all of them to 10) in Year 1, the other does them in Year 2. The target for reading in both schools was ORT 8 by the end of Year 1. That said children continue to come in from state schools and fit in quickly. I don't think that you can judge the preprep as better just because it is private.
My 2 school runs (well 1 slightly longer run) is a minor inconvenience knowing that each child is in an environment in which he will thrive. For me the main benefit of being able to pay is so that I can choose a school that is best suited to my child, and given that my children are quite different, it has turned out that different schools suit them.
I would be careful of relying on reputation alone. Make sure that you see the school at work (not an open day), and spend time with the head and see what they are like.