Given both do the ring and most tracks in the same time, same in a straight line-you would really have to like the look of the R8 over the m3 to spend the extra money. Used is another story as you can buy an older R8 but for the most part they are much pricier.
THe R8 V8 is the most expensive car that the m3 equals. I personally am going to get a v10 if I am going to spend that money or atleast a car wtih significant improvement if I am spending 30k more new.
I also would not ever want an audi 4.2 V8 they have been using with the carbon build up. Look at r8 or rs4 dyno's and they dyno really poorly with even 10k miles on them. 310 at the wheels.