In most cases it is best to burn at the rated speed as that is what your drives firmware generally has the best write stratagy for. There are some exceptions of course. Really poor quality media sometimes isn't good enough to burn at it's rated speed and must be burned slower (and still might not burn that great). Very high quality media can sometimes be burned faster (if the drive and firmware support it well enough). There are a few issues with burning 16x media at full speed (probably because it is the newest media and may need a little more development of the drives I'm guessing, not to mention, 16x burning is the hardest to do for the media and the drive). Overall though, the rated speed is best.
By the way, sony can be very good media. Use dvd identifier to tell what it actually is.