A=Average, S= Superior. Get out there to a store and dig around to get an S. I heard that they don't mark the grade on the backs anymore though, so get the salesguy to set it up for you and see if you are satisfied with it. Doubt you would see a difference though.
You would probably be pretty happy with the LG. I have an Acer with 5ms response time, 19", and couldn't be happier.