Hey canadave, my two cents...
I have a 2016 Platinum with 20"s and have driven a 2010 Murano with 18" wheels. While not a perfect comparision, I did not notice any difference.
While the common feeling that the 20" wheel size (regardless of vehicle) will result in a harder ride is true, I think it is because the 20" size is often accompanied by a low profile aspect ratio tire...often 40 or 45. The Murano bucks this trend by supplying a 55 aspect ratio tire on the 20" wheel size. This leaves a lot of sidewall height. (The 18" size provides a 65 aspect ratio tire; positively skinny!)
Another point of reference I have is coming from a 2012 Maxima with 18" 45 aspect ratio tires. I find the Murano with 20"s much more compliant.
Like you, I was initially concerned with degraded ride quality with the 20"s on the Platinum. That fear was totally unfounded. The enjoyment of such extras as intelligent cruise, cooled seats, LED headlights, power rear seat back return, etc. has made the increased cost of the Platinum edition, for us, totally worth it.
And, as a last note, the Platinum model also provides Forward Emergency Braking and Forward Collision Warning. Lately, I have thought that perhaps I'm not infallible or invincible after all, and this new technology is very much appreciated.
Good Luck!
noboy