I want to just put the numbers out there for informed discussion. A lot of people seem to have the wrong idea of the cost and potency of each Beast Gauge ability when discussing WAR.
Let's break it down:
Upheaval is 300 potency for 20 gauge. That means it is 18 potency per gauge after factoring in Storm's Eye.
Onslaught is 100 potency for 20 gauge. That means it is 6 potency per gauge after factoring in Storm's Eye.
Fell Cleave is 500 potency for 50 gauge; however, you have to factor into your calculation that Fell Cleave is on the GCD so it has an opportunity cost. This is where a lot of people fail to see the entire picture. The calculation for Fell Cleave is as follows:
If you are doing Eye-Path-Path, you gain about 9 gauge per GCD and are doing an average of 198 potency per GCD. That means Fell Cleave's real potency is only 302 with a cost of 59 gauge for about 6.1 potency per gauge after adding in Storm's Eye.In other words, Onslaught and Fell Cleave are almost exactly the same potency/gauge. Fell Cleave is simply a faster gauge dump so it's more optimal to use, especially under the effect of IR where it does overtake Onslaught for potency/gauge. When you've factored in the 5% buff from Deliverance, one gauge is roughly 6.3 potency in Deliverance assuming you always keep Upheaval on cooldown.
This is actually important because it can change the optimal rotation. If you have low SkS it's possible Butcher's Block has a place in an OT rotation, but everybody is caught up in the Fell Cleave meme (including SE, because I think that's unintended).
(There's actually another aspect to this in Enhanced Infuriate, but I'm leaving that out because as long as you use your 6x FC you should get the extra Infuriate for your non-IR Berserks)
In Defiance, Inner Beast replaces Fell Cleave. IB is 350 potency and ignores Defiance's damage penalty. SE/SP/SP has an average potency of 158 in Defiance so Inner Beast is 3.9 potency per Beast Gauge after factoring in SE. Onslaught is 4.8 potency and Upheaval is unchanged as it also ignores Defiance. In Defiance, therefore, Onslaught is actually a potency gain over Inner Beast (though IB has other benefits!).