Barbara, I believe it's a personal preference. There is a lot of information available. When you go to an agency presentation, they will try and sell you on agencies. Attorneys will do the same when you call their office.
My husband and I went to a 6 week series "pre-adoption support group" put on by our local RESOLVE group (if you are not familiar with them, they are a wonderful resource on infertility and issues involved with it.) What we liked about that group was that they weren't "selling" anything and offered an unbiased approach to making a decision. We were with other couples in the same situation. I was very impatient at first thinking that I didn't want to WASTE precious time going to a group like that.....but it was a very wise decision for us.
We ended up going with an agency for the support and the fact that they are known for their good classes, open adoptions, and lifetime support for all involved parties.
Best of luck to you in your journey!!
Mary