I made the transition from nanny to daycare when my son was just over a year old, as our nanny was relocating with family. There are bad daycares out there and there are some good ones. I chose daycare after interviewing new nannies and finding that we truly had a gem (I asked for non-smokers only and one woman showed up smelling like an ashtray). The problem I saw with nannies is that there are no checks and balances that you have in daycares. I also checked into inhome daycares, but was concerned with the caregiver wanting to take the kids to the mall and she had teenage sons living in the home and there seemed to be alot of different people in and out of the house during the day. The place was also kept nearly as tidy as I would like for young kids that spend alot of time on the floor.
I chose a daycare near my office and since I was breastfeeding, I went by everyday at lunch. Your daughter is older, so it might be more difficult to check up on her during the day. But, I found that with the daycare near work, they know you can drop by at anytime. I also worked to establish a good relationship with the care providers and chose one that had a 5 to one ratio for the younger kids. I did see some situations that upset me and made the director aware and they were corrected. I would suggest checking with other mothers in your office to see if there isn't a nearby daycare that they use.
I don't think there's a standard answer in terms of nannies being better than inhome providers being better than daycares. Each of these options has their own share of horror stories in the news, as well as distinct advantages. I would suggest you evaluate all alternatives and choose what seems best for your situation. I found the educational focus of most daycares very appealing for my kids and they were only in two different daycares through first grade (I switched them when the older one was ready for pre-K and the younger was just over two). So, with your girl's ages, you should be able to find a daycare that will meet your needs for the next 3 years.