My child had a nanny from age 3 months to 15 months, then started day care. I've had experience with both, and if I had to do it all over again, I'd choose day care over the nanny. It's not that we didn't have a good nanny - quite the contrary. She was very capable and caring, and devoted all her attention to our child.
But a high quality day care center is better than a nanny. Here's why:
1. Some of the best day care centers (like Bright Horizons) have a nurse on staff. This made us feel a lot better, especially during the choking-hazard phase. Most nannies have no medical training.
2. Day care centers follow an established curriculum, and employ teachers with certified educational training. There are no educational requirements for nannies, and most have no degree qualifications. Most don't develop any kind of curriculum appropriate to development, but are simply "watching" the kids.
3. Day care centers don't let kids watch TV, and rarely show videos. Kids at home with a nanny are likely to be watching TV. You can ban it (we did), but if you aren't there, how do you know?
4. At day care, kids are almost never alone with any particular adult. The classes are staffed by two teachers, and there is a director, usually a cook, sometimes a nurse, and other staff. This brings accountability, so it's safer. For example, your nanny might fall asleep during naptime and fail to wake when your child dones, but that won't happen in day care.
5. Peers close in age are important for development. When my child was 15 months old, she learned to eat soup from a bowl with a spoon. I would never have done this at home, but she saw the slightly older kids doing it and imitated them. Potty training, language, socialization, and more seem to be easier among a group of kids close in age. (Day care centers group kids by age.) They learn from the slightly older kids and teach the slightly younger kids (which helps with socialization).
6. If your nanny quits, your child's life gets turned upside down because it's such a dramatic change. At day care, if a teacher quits, it's less of a disruption for the child. The other teacher and staff are still there, along with their peers.
7. There is generally more accountability at day care than with a nanny. Teachers have to follow the rules of the director, who provides oversight.
8. Children in a rich peer setting make close friends, even when they are very young. When they spend their weekdays with peers, they develop strong relationships.
I keep hearing people say that a nanny is better than day care, and I once thought that myself. But it's simply not true, for at least 8 reasons.