[Serious]

I drove both of these cars recently and would go with RS5.

'21 RS5 has an updated interior that feels higher quality and the infotainment system is so much better than the Idrive 7? version in the g80. The g80 interior really is a let down from an ergonomics point of view. The button layout makes little sense, that version of idrive isn't intuitive like previous versions were, and haptic buttons near console have a vague feedback that leaves you checking the screen to confirm the change.

I also found the RS5 to have a better ride on normal roads. The G80 isn't awful but it introduces a lot of vertical motion to the passengers. I know a lot of people will find the suspension on the g80 to be perfectly fine but IMO it's overly stiff for normal roads. It's clear they've made a compromise for lap times here. I found the RS5 and it's additional compliance to be much nicer to live with where these cars will be used 99% of the time.

I think the exhaust sound is about the same. I'd be happy with either one.

G80 comp definitely has the edge in roll on power but the gearbox feels lazier than the RS5 at high rpm full throttle shifts. Shifts under 5000rpm it feels great.

Rs5 obviously looks better than the g80 but I actually still like the g80 looks enough to buy one. Sat next to an f80 CS overall I liked the g80 more. It makes a statement the f80 can't despite the odd grills & front end (much better in person than photos).

Where G80 has real advantage is on price and standard build/ options. I would be perfectly happy with a base G80 comp for ~73k MSRP. Audi charges a ton of $ for options that are standard or must adds on the RS5. My minimum build was $82k and that didn't even include $1500 navigation the dealer wanted me to spec. Audi wtf are you doing charging $1500 for navigation in 2021?!?!

Despite being older and significant more expensive I would still go for the RS5 it just feels more special and cohesive than the G80 comp IMO. I think in time the G80 comp AWD face lifted model will fix some of the issues I found but the current car isn't there yet.