The staff at Occaisional care centres all need the same sorts of qualifications and/or experience as do staff at other types of centres. I do tend to agree with dachlostar that they would be more likely to have higher standards than private, BUT that is not to say you won't find an excellent centre that is private.
I would go and look at the centres, spend time there and see how your DS reacts with the staff and children there. Then go from there. Good luck!