To my experience, the difference is as follows:
In 2D client, when you have lag (due to many objects on the screen, bad connection etc.) it takes you longer to pass to the next tile. The 2D client steems to calculate one step after another.
In 3D client, lag doesn't seem to affect moving speed very much. It seems to be more lag-tolerant. Maybe the route is calculated differently. I think that lag doesn't influence moving speed very much in 3D.
This doesn't necessarily mean that 3D users have an unfair advantage. It just means that lag issues just don't weigh that much as in 2D. It CAN be an advantage.
I switched the provider a few weeks ago, and now I am lagged terribly on Catskills (200-300ms ping). With KR client I can move much easier and faster.
Of course, KR client has a better pathfinding algorithm, too. That's a little advantage when you move through the woods or the jungle. However, an experienced 2D-user will know how to move around trees in the fastest way.
Conclusion:
With a good internet connection, a decent computer and experience, PvP between 2D and 3D client users is pretty balanced. KR client users still have to fight all the bugs, don't forget that!
