I've worked in a day nursery and now work as a childminder and without a doubt would say choose a registered CM.
Reason 1: Your little one will be spending the day in a normal family environment not in an artificial setting/institution.
Reason 2: When I worked in a nursery I worked there for 4 months whilst waiting for my police check to come through (CRB), in that time I was constantly given sole access to babies from 3 months up even though this is against the law. Believe it or not this is not an isolated incident and parents were assured that all workers were CRBd! CMs are not allowed to work until they are CRBd
Reason 3: Find a CM that you gel with and that is who will be working with you to care for your child for the next xxx years. In a nursery there is a good chance that the 'keyworker' will be changed randomly and regularly which will upset your child.
Reason 4: Most childminders will take your child out and about in the community, he will get to know the shopkeepers, the community police, visit the schools, playgroups and libraries and interact with the children he will later go to school with. In short he will have real life experiences with real people.
Reason 5: Your child will be one of three (or possibly four) under fives, not one of thirty so will receive much more one to one care and attention than in a nursery - with the same person encouraging the same standards of behaviour which have (usually) been discussed with mum beforehand. Even the best nurseries, like schools, have to offer a one-size-fits-all because it's simply not feasible to work to a parent's requirements in a large institution.
Reason 6: Pick the right CM and she (or he!) will become an important part of your life. Many CMs will work above and beyond the call of duty as long as parents don't take them for granted. Your CM will grow to love your child and care for it as if it was her own. Key-workers are often moved around IME meaning the child they care for then has to form new attachments to a new key worker which can become very disressing.
I loved working in the nursery and most children were very happy there so I don't want to disrespect the services they offer, I just feel that CMs can offer a more 'natural' experience for young children.
In reply to a previous post: Yes we do have 'off' days, we are human, just like nursery staff, lol! ;)
With regard to having four different carers, your child will possibly not settle in any of them making them fractious and upset. I'd go with just the three, either Mum Gran and nursery or Mum , Gran and CM
Good luck with whichever you choose x