I employed 1 childminder for 3 weeks, employed 2 nannies in shares for just over 4 years, and have done a placement in a nursery. I'm extremely biased and will say right out that I would have given up work rather than put ds into most of the nurseries I have visited. This is not meant to diss people who choose otherwise, it is meant to show how irrational I am on the subject.
I will base the pros and cons on a full nannyshare outside your own house - i.e. you are taking your child somewhere else to be looked after with another child(ren).
Pro:
Nanny can set as much, or as little, of a routine as suits them and the children.
Nannies are expensive - because they are only looking after a few children (2 in my case, my ds and the child of the share parent). I employed a wonderful childminder briefly, but she had what felt to me like a huge range of children she was looking after - before school, after school, different ones each day etc. As I say, she was wonderful herself, but I found it unsettling to continually be meeting new children - and some of them were not interested in being nice to small ones like ds .
Nannies are expensive and you have a full employer/employment relationship. It may seem odd to put this as a pro, but where my child is concerned I like to be able to call the shots within reason. OK I'm now sounding like a nightmare employer, truly I'm not. But because I was the full employer, the nanny's employee relationship was explicit and ds was our joint priority, I could make the rules I felt were important without having to consider other chidren and their needs much.
Con:
Sickness. Both the nannies I employed needed quite a lot of sick leave - one had to take a month off to recuperate from an operation. In my experience, the nanny WILL need to take at least one day off in the first week you go back to work, when it feels like a catastrophe! You need backup, and where is it going to come from? Not easy. In our case, grandparents, and I also just had to take time off. DH was too ill at that time to be able to offer much backup. You may want to look into sick leave/maternity leave insurance. It's really important to accept this if you do want to employ a nanny, because sickness can and does happen to anyone.