Not sure, check the forums on the C2 website. They have a lot of good info.
I just bought a C2 model D(about a month ago). The level of resistance is more than sufficient. Search the forum and read everything Chris Wyant has written, also check the Crossfit Live on the C2. Reams of good info. I have been spending a lot of time on my new toy, and have PR'd every distance since I got it. Also look into the drag factor settings, which are actually more accurate and transferable than the damper setting.
Also, the 500 is a completely different monster than the other distances. Shorter, faster, harder strokes. Gives the fan less time to slow down, you have to pull less hard to keep it at the same pace.
It has been well worth the investment.
You can also do workouts that aren't your standard timed distances. 3 sets of 10 100m sprints, 10 stroke max effort pulls for max watts/pace, row/pushup/situp triplets...
my workout log details my progress:
Most of the workouts are taken from here:
the following SFW links regarding C2 and rugby training: