I took a cursory look at the scaling coefficients for SF and DS, and came to a couple (rough) conclusions.
First, they scale identically. That means they get the same benefit from SP, have the same coefficient, and obtain the same benefit from haste... up to a point.
However, they have a couple differences (though correct me on these if I'm wrong, I haven't been able to get on beta in a couple weeks):
SF always crits. Crits are 2x damage, meaning SF always does 2x that of DS.
SF benefits from crit by gaining 1% extra damage per point of crit (from crit rating only, though).
SF has Molten Core.
Now obviously Molten Core procs are a bit of a game-changer. Halving the cost and casting time of a spell is pretty big. Even with 0 crit and 0 haste, SF gains a higher DPET than DS (2s cast vs 1.5s cast, 2x damage vs 1x damage). Factor in crit, and it's always a better way to spend your time casting SF while under MC than DS.
Also with 0 crit and 0 haste, while under MC, SF and DS have identical DPFC.
Now, without MC procs, that's a different story. It still might be advantageous to burn off all your fury as rapidly as possible... You might be casting SF at 4x the cost of slash, but under high haste situations (procs/lust), it might be better to burn it off and start generating more than to squander that haste on a GCD-capped spell. It's important to remember that while in meta, any haste over 50% is entirely wasted on DS. Also, with higher haste and crit, it might actually reach a point where SF becomes equal to DS in terms of DPFC or DPET (or both) without MC. I think those numbers might be so high they're unrealistic, but that point certainly exists, especially considering DS's hard cap of 50% haste.
Now, again, I haven't been able to play with the beta for a couple builds, so I'm just going off information scraped up here and elsewhere. Correct me where I'm wrong... and if someone could give me the real, current numbers (scaling, whether or not SF still scales linearly with crit), that'd be greatly appreciated.