I chose a nanny for when they were young then dd went to a nursery about 3 as nanny was off having her own baby.
As a carer for a baby I definately think a nanny is better.I don't like a nursery environment for a very young child personally. For your 3 year old it is different but I presume he would still go to nursery school/playgroup in the mornings and then it would give him time at home also with his sister.
Advantages of a nanny:
Your children get to bond with one key person and you can individually pick this person instead of the nursery.
You leave them in their own house - only the person looking after them changes.I personally think this is the most important thing as a baby.
You decide their routine and the nanny sticks with it.
You decide what they eat.Nanny will cook it.
Nanny will also wash their clothes.
They get to sleep in their own cot for naps and aren't disturbed by other children.
You don't have to get them ready and out of the door in the mornings.
If they are ill the nanny can still look after them and you can work.
More flexibility on hours.
They can still socialise as much as you want most nannys take them to music,toddler groups - whatever you want.
Most play is pretty solo/parallel play until 2 1/2.
Dd went through this care and so is my ds now.Dd is at school now and very bright with lots of friends so I don't think was disadvantaged in not going to nursery as a baby.