If you are comfortable sailing a boat but are trying to develop better sail trim and boat handling skills, I strongly recommend J-World''s skills building programs.
As near as I can tell, there is little difference between ASA or USSailing certifications schools. I have never been through either but talking to people who have they each seem to have minor strengths and weaknesses. One thing that came out of a discussion last spring is the huge differences than an individual teacher can make. Sailing teachers generally are not trained educators. Some are naturals as instructors but some really do not have the knowledge or teaching skills to convey the material well. An acquaintance described an experience starting a sailing course and then getting sick on the third week. The school agreed to let her drop out and then retake the course when she was well. She described the differences between the two teachers as ''night and day'' and got a lot more out of the second teacher. So if possible, you might ask to audit a class with the teacher who will be instructing you to see if you are comfortable with their style. (You possibly may have to pay a fee to sit in on a class.)