I think you should sit in a coupe
That's what I did when I visited FFR. I was pretty sure I wanted to build a hotrod but after sitting in all 3 (GTM was over budget and 818 was not out) the coupe just felt right.
Even though the foot box is deeper so your knees are not bent up it is narrow as the 33 pinches in at the nose. Also, visibility with the hard top is not very good. I am pretty sure the ride/handling is very similar to the roadster. I have also heard that they are louder with the hard top. You might want to PM Tom Veale as he has had both.
Take a look at Rumbles build log if you haven't already:
He paid much attention to sound control and documented his build well. Pretty sure he gave some driving impressions in there some where...