IMO you will find the 2015 Plus 8 an much easier drive than the Roadster. the new Plus 8 being based on the Aero chassis is a much more refined and controlled experience lending itself to more power. I have driven both the new Plus 8 and used to own an Aero S1 and Roadster 3.7. The Aero which is very similar in feel to the Plus 8 was a great ride, I have also been a passenger Peter J's Plus 8 and it compares well to a pure Aero feel.
Hope this helps.
