How do I decide whether to hire a nanny or put my baby in daycare?

Tough choice. Nanny is better if safe, reliable and affordable. Day care is better for peer socialization, but more likely to increase the number of infections during early childhood.
Carefully . Aside from cost issues(both can be relatively expensive)consider if you want your child to be raised by one additional person or by a staff; in your home, theirs, or in a group setting. Keep in mind the latter option (daycare) will typically expose your child to more cold/ flu illnesses. Make sure credentials and first aid skills are current for both considerations.Carefully consider your options.