On my 07 316 m sport convertible I have nice 20” wheels rides a bit hard but I love them , but bought dec 08 330d m sport auto convertible with 18” wheels no fussy on them so was thinking before selling the 07 of swapping wheels. I think in general the 08 dots s we bit higher and may be softer ride, would the 20” totally ruin the ride in the 08.  
photo of my 316 with nice wheels shown. Also tyres are all new on the 20” only selling my 07 as wanted to change to auto. It’s a pity as it’s showroom condition throughout demo then 1 prev owner from new. 

Hi

If you've got both cars at the moment, swap them and see which looks best / drives best. People would give their opinions of pictures but you have to be comfortable with the ride and handling.

Hope it works out well for you.

John