I have been a nanny and have put my own child in daycare at one point. As a "nanny" and as a parent I would say that a nanny is a more stable, personal situation for your kids - your house, their beds, etc. It's a great "second" to having Mom or Dad there. At daycare you have no control over the turnover, dealing with other kids, etc. Not to mention that kids share all kinds of germs and your child will be sick a lot...it either happens now or when they enter preschool/grade school. And our ped even said, better when they enter school age as they can deal with it better and are better able to physically fight illness. Besides, if you do "daycare" any time your kids are sick (fever, vomiting, etc.) YOU or DH will have to stay home with them as they are not allowed at daycare or preschool.
I think as long as your older child is already going to attend a preschool program he will get the socialization and education that a daycare preschool could provide.
If you were saying EITHER daycare w/preschool or home all day with a Nanny, my view would be different. Since your child is already going to preschool and getting those critical exposures to a school environment anyway, I highly suggest sticking with the nanny. Esp if you feel this is a reliable person who follows your rules/expectations. Good nannies are hard to come by. (Just CYA and have a backup plan in case your nanny ever is sick or bails on you. At that point, you could reconsider daycare.)
Best wishes.