If you are willing to use Into The Fray, you will find yourself generating fury much faster to start slamming.
couldn't have said it better myself.
aftershocks of Rumble is spread out over 2 seconds and you do an additional 2 procs (of Into The Fray and LoH) in that 2 seconds. So although Rumble returns more Fury and LoH, you actually have to wait 2 seconds to get the full benefit. If you have faster attack speed, you will have more Rumble effects underneath the mobs at the same time, returning Fury and LoH faster.
This appears to be why rumble is so good.
But, Mr. Orion, can you compare Strength from Earth vs Rumble?
My attack speed during Wrath (and I'm 99% in Wrath during a run) is around 3.5 APS.
For smaller sized mobs and one-to-a-few elites, I find Strength from Earth easier to spam. The 30 fury cost of Rumble is quite a bit especially because of my stacked attack speed.
Currently, my technique is to alternate Weapon Throw's periodically (as I have -10 fury WT's, they only cost 1 fury and NoEscape+IntoTheFray = 14+15=29-1= 28 fury generated per crit (and my crit chance is almost 70% in wrath).
So, my slams and weapon throws basically become a blur.
Now, with Strength from Earth, I don't have to alternate WT's nearly as much.
If comparing Strength from Earth vs Rumble, in an extreme situation where you have infinite Fury, then Rumble will always be better, as it does more damage for the same attack with the same attack speed. But we don't have infinite fury, so comes the question whether you can sustain the fury for Rumble, if you can, Rumble will still be better. Whether Rumble can be sustained depends on the mob size. At your Crit Chance, you can sustain Strength From Earth with a 4.29 mob size and Rumble with a 8.58 mob size from the initial slam (not factoring in the additional fury return from the Rumble aftershocks as that will make the calculations more complex).
But if you're talking about getting the full benefit of Rumble and it's aftershocks and assuming the mob stays in the aftershock area the whole 2 secs without dying, a mob size of 2.86 will return you 30 fury enough for one Rumble slam, but you have to wait 2 secs for it, so it's actually not sustainable.
The above is just some number stuff for your consideration, in actual game play, the bigger the mob the better for Rumble, very obvious point. Problem is although your attack speed is fast, Rumble aftershocks still operates at 1 proc per second which means you run out of fury faster before the additional procs return fury for you. The faster your attack speed, the more likely this will happen for the first second when you start engaging a smaller mob. Maybe that's why you find SFE better.
In my guide, I wrote something like choose SFE if you find yourself killing mobs and elites in about 3 seconds. In your case, if you find SFE is better for your attack speed vs a small mob size, then you can use that. But I still think SS should be used more for AOE which means its meant for larger mobs, and Weapon Throw to handle the single or few targets that are left. With your -10 fury for WT, I think generating fury for Rumble is not a problem and will definitely be better when you need the additional damage on huge mobs at higher MP level.