It depends on many things.. How good your local state schools are, whether you can envisage your child happy there. How well you could afford to send your child privately. What the private schools are like, what you plan to do at senior/secondary school level.
My DDs both went to state infant & junior schools and then onto private Senior school. Firstly because when they were younger I could not afford prep, but also because the state schools they went to were very good. My eldest DD was unhappy at the state secondary, so we moved her, and subsequently her younger sister to a private school.
DS is about to start at the state infant school - we could afford to send him privately, but like the infant school so he will go there. We will look at sending him to Prep at 7, even though the local state junior school is very good, primarily because there is a shortage of Boys' Senior Schools locally, and we want to maximise his choices, and the Prep will do that.
Advantage of a good Prep over a good State school tend to be smaller class sizes, and better facilities - whether it is worth paying the extra depends on how easy that is for you and how important you think those things are.