One butcher's block combo generates nearly as much aggro as 5 fell cleaves, so it's not that simple. Also if you wait to build up enough gauge for the 6 fell cleave rotation you'll have to start with eye-path-path-eye combo before using berserk, and that's like 25-30s into the fight before using your fell cleaves.
Upheaval is better than fell cleave in terms of potency/gauge, and it's off gcd so it's better. The only time you shouldn't use upheaval is when you'll lose a fell cleave for a heavy swing or maim during a buff window (500 > 150/190 potency loss is bigger than 300 from upheaval), otherwise upheaval takes priority over fell cleave. Inner beast is a defensive utility so obviously it takes priority over the other two, assuming you need it.
Onslaught isn't good but there might be cases where you can spare 20 gauge when upheaval is on cooldown, so that's still a "free" 100 potency. For example between berserks your goal would be to build at least 120 gauge so you can use one upheaval out of berserk and still have 100 gauge to start the next berserk. If you can build 140 gauge instead, then that 20 extra gauge can be spent on onslaught without delaying your berserk. I did some experiment on a spreadsheet, and it seems like I can use onslaught once between my 2nd and 3rd berserk without losing any upheaval or fell cleave (since using a fell cleave after 2nd berserk before 3rd berserk would delay your 3rd berserk by 3 gcd).
