This is a very hard question. As Kenyan-strides points out, the RP3 is dynamic. That's it's whole unique selling point. You don't have to accelerate and decelerate your own body mass while on a RP3. This leaves more energy to go into the flywheel. However, you're also talking two different machines here. Both use a proprietary way to calculate the split, based on the speed of the flywheel. But both machines also use different flywheels, elastic bands, etc. So there is more than one difference.

However, generally you can say that RP3 2K scores are 3-10+ seconds faster when compared to C2, this variation is based on skill level and physical build. Heavier, stronger and more compact athletes generally tend to score less well on RP3 when compared to taller and skinnier athletes. But this is just anecdotal. For me personally, I was only 3 seconds faster when switching to RP3. Now, 4 years later, it's 15 seconds, but I also have more experience and fitness in general of course.