A faster OH weapon can produce more tornado ticks, but will also cause WW to drain your fury quicker. I'm not sure what the optimal balance between the two would be, but a higher critical hit chance would certainly make it easier to run a higher APS.
Fast offhand will increase the average DPS of your sprint, which is much more damage than your whirlwind, so even if it means you stop using whirlwind for a sec every once in a while you should probably still use a fast offhand. Of course, if you have a slow offhand with much better stats then just use that, but optimally use a fast offhand that also has good stats. Offhand DPS is less important because sprint always uses main hand weapon damage (but alternates weapon speeds as you whirlwind and the APS on your character screen alternates between the weapons). It's a (very) messed up mechanic but it seems like that's how it works.