The Cowbell is held at Middle Creek campground, just above Clear Lake. The WFO is held at Fouts Springs (aka Stonyford). The WFO is run on the eastern side of the Mendocino mountain range while the Cowbell is run on the western side of the same range. The topography for the WFO contains some steeper, longer hill climbs/decents when compared to the Cowbell. Both events provide challenging single track trails. The Cowbell is run earlier in the fall, and is usually dusty where the WFO is later in the year, and usually has wet conditions.
In my opinion, the Cowbell is the easier of the two events by a slight margin. At least that has been the history, with the WFO having some fairly sneaky timekeeping with some really tight sections thrown in for fun. The person who has laid out the WFO in the past will not be doing it this year, so I am not sure what the personality of that event will be for this year. Both events are fun with the weather dictating the difficulty of the trails. I might add that the Middle Creek area can be a real challenge insofar as water crossings are concerned if it rains. This could significantly alter the difficulty comparison of the two events.
Take your pick. If you want more info, email me. I am pretty up on all the enduros in D36. I hope you decide to try one, they are both great rides.