i probably have a biased opinion because i am a nanny, but i vote nanny for the one on one care you get. they both have great pros and cons.
Personal, a "sudo" family member, some one who knows the ins and outs of your baby, what soothes her or him, favorite crafts, books, sticks to whatever schedule you provide (nap time, bath time, feeding schedule, allergies, ect) nannies give a variety of things you might not be able to get from a daycare, like language skills, working on milestones like walking, or crawling, and give individual attention vs a class setting. Another bonus* Nannies also do house work, most always. It would sure be nice to come home and the dishes were done and all the toys were picked up and put away when the baby was sleeping. :)
Children get to experience a group setting and learn more things like sharing and get to learn via a school method, where as a nanny would just take things on as they come, a daycare has a planned curriculum, -monday they learn colors and shapes, tuesday is crafts and numbers, wednesday is swimming and sharing. ect.
My personal experience goes to nannies, thats my bread and butter, but we have very close friends that swear by their day care center. both of their children 3 1/2 years, and 17 months have been in that daycare since 3 weeks, and it shows. They are very advanced, and especially the language skills of the 3 1/2 year old are exceptional.
if you do end up conceiving, i use care.com to search for families to work for and they search for nannies, babysitters, occasional care, mothers helpers ect. let me know if you need any more advice.
we are ttc as well. Good Luck ! :)