Re noticing and/or telling the difference, facts: to notice the difference at all, the desync has to be 100ms or longer. This is due to the human eye being incapable of noticing motion changes faster than 1/10th of a second. At regular framerates (24fps), this equates to 2.5 frames. As for the difference in quality between the encoders, nobody can tell based on the same bitrates or whatever. The only real difference between encoders in that regard is compression efficiency (how good the quantizer is), and what the clipping ceiling is. While the quantizer is not tuneable in this manner, the encoders are generally close enough that there is no real difference at reasonable bitrates. Clipping however is tuneable and a set of headphones with a reasonably good dynamic range can show the point quite clearly.