You have to write out a pros/cons list for yourself after you meet with the agency. Depends on what YOU want.
Personally, I would NEVER go with an agency, but maybe its because I have the experience from working in a variety of centers. I have seen what my nephews' one provider went through, and didn't want that for myself.
IF you go with an agency, you have to adhere to THEIR rules! I don't know how it would work if you went private and with an agency for numbers; but I'm going to assume that you will have to abide by the agency's numbers. So 1 under the age of 1, 2 under the age of 2, etc. Whereas if you go private, YOU decide on what age groups YOU want!
I found with agencies, is that usually (and I know it's not always the case) these are families that no one usually wants. They're the parents that don't listen, and the kids that are little shits.
Again, it's all up to you! Good luck!