I think they are quite comparable, so either one really works. What is probably much more important is WHO you learn from, and how much time you get to spend on the water. The quality of the boats is somewhat important, but initially even that isn''t as critical as the ''who''. If you are totally new to this, you might want to consider taking a dinghy class prior to big-boat stuff. Here in the bay area, Berkeley has a great little program. You learn a lot, and it''s WAY cheaper than the big-boat stuff.
I think that before deciding which certification organization that you want to go with you should probably decide what type of sailing you are going to be doing.
US Sailing really is into small boats and dinghies and racing boats of all sizes. Only recently have they really started to push themselves into the cruising realm. With good reason, they are really good at what they have been doing for years.
ASA on the other hand puts more emhasis on the cruising aspect of the sport, and is really weak in the racing or small boat realm. Just recently have they started pushing their small boat program.(I know this because I am an ASA instructor) Just look at the names of the first few ASA course levels and they pretty much lay it out for you. Level 1 Basic KEELBOAT Sailing, Level 2 Basic Coastal CRUISING, Level 3 Coastal Navigation, Level 4 Bareboat Charter, and Level 5 Advanced Coastal CRUISING.
I agree with the previous post, who you take the class from is equally important as which organization that you use. Get student reviews and talk to the instructors see how they like the facilities, boats, and curriculum.
You will benefit in the long run by using one of these organizations because they will provid you with a standard that is recognized all over the country and abroad. I hope this helps.