This is one of those there-is-no-right-answer questions. It depends.
The proof is on paper, for max acceleration, the A8 wins. And in traffic, yea the A8 gives your left leg a break (but who really needs a break unless you have a leg problem).
In terms of which gives more enjoyment, that's pure opinion.
The first HC I drove was my wife's Charger A8. An eyeopening experience no doubt. I drive it intermittently and it is a monster or tame depending on my mood. I have used paddles in old CamaroSS and some with her A8. Think with more time I might like paddles more, but tend to drive in auto. It goes hot when you ask, but from ptA to ptB, even in a HC your mind can wander to work, politics, etc. and mute the driving experience.
With the M6 in my ChallengerHC, you are always involved. Every shift, timing and quality, depends on me. Some shifts great, some not so great, either way its all on me. A perfect manual shift, up or down, is actually smoother than auto and very satisfying. It may not be as fast as the A8, but on Continental ECS 305's on the rear, you can still spin em at will thru 1st. I'm not bored yet. With used prices as they are, I might own a Redeye by now, but no M6. So I'll keep what I got, happy as a freakin clam.