I agree about it being the staff that make the centre. DD goes to a fantastic centre (which just happens to be an ABC). You need to pick a centre that you and your son feel comfortable with.
Things that were important to me included the children being happy, the centre being clean (but not sterile), the friendliness and competence of the staff.

On the sleep thing - try not to worry about it too much. As someone else said, the carers are experienced with and used to sending children off to sleep. If you are worried about it, you could send something with your scent on it to be placed in the cot with him. I send DD's comforter (a mini blanket with an animal head on it bought from Best and Less) each day as she sleeps with it at home.
And don't worry about being anxious. The staff are used to it and we've all been there. I was beside myself - would these people take as good a care of my daughter as I would? My DD LOVES child care and loves her carers - and they love her. And they can settle her just as well (if not better) than myself!

MG