There's an article in this month's Bike (or Superbike). Both are British mags sold here in the States. They put them alongside eachother and compare. I see you're from the UK so check your newstand!
Having ridden both at speed on the track, the RC8 feels softer but much faster. More comfortable and refined too. The RC8 is also less cramped as you will read in that article. I want one in the worst way, but another RC51 will have to do for now. Same linear power, but the RC8 pulls you up to warp speed, comparable to any other modern liter bike. It's a pretty amazing bike to ride, light, agile, friendly, and powerful.
KTM used to put on trackdays at Mid-Ohio (AMA track) with 20 bikes, nearly all brand new RC8R's. Show up, the bikes are gassed and ready, and go ride all day with standard group sessions. The best part (besides being on RC8's) was it was only 100 bucks more than the average trackday. Unfortunately they pulled the plug for this year though.