I'm the Executive Director of the ASA, and wanted to throw in my 2 cents regarding ASA and US Sailing.
Certification, and the education and practice that goes with it, can be extremely valuable. It also evidences that you have achieved a given level of proficiency, which is helpful when chartering.
However, learning to sail and PRACTICING should be one's goal. Some people get certified and then don't go sailing. That's not the point. Certification means little if you learn in a week and don't get out on the water for a year or two. That's why diving facilities, for example, don't just ask "Are you PADI or NAUI certified," but "When is the last time you dove?"
ASA and US Sailing standards are nearly identical. Both systems have many excellent schools and instructors, and each system can claim "bragging rights" over the other in certain respects. US Sailing is the official amateur racing body for sailing in the US. ASA has more sailing schools in its network and was the first to introduce keelboat standards to the U.S. But the most important consideration is the quality of the particular school and instructor, not "which system is better."