If you plan on doing a lot of track work, you'll want the SC. The extra power is very much welcomed on the straights where the R gets a little breathless.
As a weekend fun toy, I much prefer the R. You have to work the gearbox and engine more, but that adds to the pleasure rather than detracting from it. With the SC on the road, you either end up using one gear and the higher torque to propel you or you go so quickly that losing your licence is a real possibility.
I'd rather get an R and spend the extra on some Exige seats or Nitrons than get an SC, personally.