Ex 320d Touring owner, now in a 330e M Sport saloon, having also driven 120d M Sport.
The main difference IMO are as follows:
Standard 320d – Comfortable, fast enough for day to day driving to use the engine hard, always able to put your foot to the floor and know the back end will be ok, and maybe just slip a little. Overall very nice.
330e M Sport – bonkers fast compared to the 320d, much better chassis control, more communication when the back end has gone and you can keep it there, back end goes way way easier ie you have to be careful how much throttle to use in corners. Too much power for day to day driving country roads, constantly braking catching up cars (if using your right foot a lot). More effortless to drive day to day, more refined than the touring.
120d – OK performance, bit skittish compared to 3 series, less comfortable ie shorter/narrower wheel base. Feels less grown up than 3 series, but not as nimble as a sports car.
You need considerably more talent to get the most out of the 330e than you do the 320d, and be prepared to go out at 5am in the morning to drive it fast, but it is still no MX5, Lotus, Cayman etc
Edit: The RS6 you would need to get up even earlier in the morning, and have to be a super talented driver to get anything out of it on the road… unless you just like hoofing it and letting the car do everything for you.
Edit Edit: Haven’t missed the touring either, bikes always went on the roof, and internal space, not that much less.