S.
M.,
When I was a first time mom of an infant (with another on the way, I might add), I was an elementary school teacher. I actually was faced with the exact choices that you describe. Keeping your child's health in top priority, I suggest the nanny situation. My first baby was prone to ear infections, and we didn't find out until she was exposed to colds and such in a daycare setting. While their social interactions may be a bit limited, I feel that you would be better off keeping the baby as "at home" as possible, at least for the first year or two.
Around the time that my first was 9 months old, and my second was about to be born (Argghh, heehee), I pulled her out of daycare to stay home full time. It was a struggle at times, but my second one was much healthier in the "contagious" category for it! I realize that staying at home is not always an option. That is why I suggest the nanny/sitter.
I hope I have helped.
S.