I had the V10 when it came out, then I got the V20 when that came out and is what I'm currently using. As far as sound quality, I thought they were both about the same (I have decent headphones, not the best. Sony MDR-Z7). But I will say the V20 is just an all around better phone.
Oh, and to the guy who is saying the LG V series doesn't have a "real" DAC. I'd suggest you do some research before you claim stuff like that. The V20 has an ESS ES9218 which is similar to what's in many desktop and USB DACs. Check out Head-Fi and you'll see most of those guys don't use external DACs with the V20. I'm pretty sure the ESS DAC in the V10/V20 is much better than what you'd find in most HU's, so you probably do get better sound quality using analog AUX in from your phone instead of USB