The original R8 has slightly more power due to changes in the rules limiting the Oreca car, however I don't think the difference in performance is too noticable
I only have the R8 01 race car and it is amazing. I did plenty of research before hand and all recommended this way above the other one. Nice livery on the 01 model too.
I won mine somewhere along the beginning but I can't remember how now. The car comes in handy later on during the German only events, easy way to win gold.
'01 version is lighter and redlines at a higher RPM. Lap times are slightly better with it, it corners faster than every car in the game except the Ferrari Formulas, FGT and X2010.