M.C.
I used both. From 3 months to about 27 months, I had a nanny/sitter in my home M-F 9 to 5. I work from home and really liked knowing I could at least hear my baby all day and could jump in if any emergency happened. In addition, I liked that he was receiving more individual attention than he would at daycare. After he was 2, I moved him to part time day care (3 days) and part time nanny (2 days). I did this because at this age, and with him being an only, I wanted him to have more interaction with other kids. Right before his 3rd birthday, we moved to full time daycare/pre-school. I don't think he was being engaged enough here (some other nanny's may be excellent in this area however). He had a lot more fun/more activities at school. His school is a block from my home so I don't loose much more time with him than when I had the nanny. I am saving a TON. Preschool is working out to less than half of what I had to pay for full time sitting. However, I only have one..so that gap will be much smaller for you because the Nanny's rate would not fluctuate as much between 1 and 2 children as the daycare would.
However, you need to do what is best for you and what you are comfortable with. If I did not work from home/had to go to the office each day, I would have had to use daycare. I don't think I could have ever found a nanny that I felt comfortable with being alone with my child all day. I was not always as like this...motherhood brought this out in me :-) I trust very few to care for him solo. Although both the nanny and daycare teacher are certified etc, I would feel more comfortable leaving him at a daycare (that I researched etc) where there are several teachers around, parents stopping in to "observe" at random times etc. than I would with my nanny who was a stranger up until a few days before. Good luck. I know it is tough and heart wrenching.