It really is a very personal choice. For us, the flexibility of a CM, plus the one on one attention when the older children were at school, the actual doing of the school run, the mix of ages, the fact she has a dairy allergy which means I literally cannot afford for her info not to be communicated properly, being with older kids and so less likely to have cross-contamination of foodstuffs, were massive factors for me.
For me, and it was very personal as the allergies run in the family, so I'm also really nervous and reluctant to send DD to school when the time comes, even though things have changed dramatically in the last 25years, it was better to trust one person and deal with the odd sick day than the chance of much more sick days and total lack of sleep dealing with DD reacting yet again because a keyworker wasn't watching when the child smeared in butter/ice cream/cream/cake/biscuit/ham/chicken etc., etc., gave DD some by accident just wasn't worth considering tbh.
H works flexibly and our CM only charges us fir the hours we use. For me it just doesn't seem right to divide kids by age, and she's involved in everyday life, going shopping, different groups, just like she would at home. For me, and just for me, everyone is different and there's no right choice, it was about providing DD with an environment as similar to that she has at home as possible.
That's what education is about for me, when I'm thinking about an 18m old. She has a caring adult, two older 'sisters', 'cousins' as in other age-mates who come to visit (my CMs older childrens children) and a menu that takes into account her dietary needs, with as much margin for error as she has at home.
I'm not looking to debate, or even to influence your decision really, just explain why I reached the decision I did.