Creche -
Lots of minders, so there is no individual minder left with child, so you are not dependent on just one person if that person is sick.
You child slots in to an established routine, and he/she sort of just needs to conform.
Child learns to adapt and be part of a group.
Child is exposed to huge amount of germs - and will tend to have more visits to doctors, and have a runny nose for 3 - 4 months of the year.
Initially child can get quite stressed when being left in an unknown place with unknown people.
Need to check on teacher to student ratio - and the level of care. Often classes are too big, especially among little ones.
Individual children's needs sometimes get lost in the masses.
Mom is often very stressed - and mom is also having to ensure she arrives at a particular time to fetch child, especially stressful if mom works a full day, and mom is running late.
If child is sick and cannot go to creche mom needs to make another plan.
Mom needs to prepare baby bag the night before to get things ready, and factor time to drop baby off at school in the morning.
Nanny -
Child remains in his/her own environment, and is not so stressed by mom leaving (or less stressed).
A well trained/experienced nanny is a god-send.
Child is not exposed to much in the way of germs, so child is spending less time at doctors.
You are dependent on one person, and if that person is sick, you are up the creek.
You are dependent on one person, so you are putting your absolute faith in that person's ability to take care of your child in the correct manner.
Child does not get play time with other children - but I would not really look at this as an issue until they are around 18 months.
Mother is not stressed as does not have to rush in the mornings or evenings.
Mom does not have to pack or prepare in the morning or the night before.
The cost of a nanny is slightly higher, but the benefits far outweigh sending a child to creche.
The key however is to find a really great nanny.
I have done both, and have opted for the nanny as (for me) it has been proved time and time again to be the better option for my kids.
As my kids get older, I have them part day at a daycare or creche and part day at home, so they get best of both.
Hope that helps.