Well, I'll put a few notes up top before I get into generalities. The first is that you shouldn't just say "I will only use sbar on this fight and only sblock on this fight". It is very important to know when it is a good time to use either one. Especially on fights where sblock is the primary ability you're going to want to use, it is not possible to keep a 100% uptime on sblock, so planning sbars in those gaps is important.
Stone Guards: SBlock, you want damage smoothing here, the most damage you're going to take is from rend, but it's steady and consistent and not that much damage in LFR, You'll probably be tanking multiple mobs and sbars will get chewed up way to fast and your damage intake will be very spikey, use sblock.
Feng: Feng is kind of a toss up in my opinion. In normal even my healers couldn't really tell a difference between if I was using sbar or sblock. I would say use sbar for phases 1 & 3 and sblock for phase 2. There's just a ton of magic damage being thrown around.
Gara: Correct, pretty much sblock all the way here
Spirit Kings: Again correct, sblock here
Elegon: If you can time an sbar with the breath attack then hit sbar then, but sblock is probably going to be better overall.
Will: Sblock is good, but if you screw up a devastating combo attack, hit sbar to absorb it so at least the healers don't have to heal through your mistake.
HoF: I honestly forget the names of these bosses, I just remember the order:
1) Sblock mostly on the first boss, there's not a ton of non-avoidable magic damage damage here. I would say use sbar as an "oh crap" button if you screw up attenuation or something.
2) Most of the damage is blockable, so you should keep sblock up UNTIL you see your overwhelming strike timer come up, right before it hits, get a big sbar up there to try and absorb as much damage from it as possible. This is really the only mechanic that can kill you and really as long as you're taunt swapping at 2 stacks it should be trivial.
3) Garalon: Nothing is blockable here, it is 100% pointless to press sblock on this fight. Try to time your sbars so that they absorb the furious swipe instead of pharamone damage ticks.
4) I think probably sblock on this, especially since at one point you tank two different mobs.
5) sblock, especially if you're the add tank, you wanna keep that up as much as possible.
6) Sblock again is going to be the best option on this boss, especially during add phases. Lots of melee damage, any magic damage is pretty predictable, as long as you're taunt swapping it shouldn't be an issue.
Council (1st boss): I've only tanked this boss a few times, I've been focusing on DPSing a lot. Seems like the first guy killed is the only one that really does significant melee damage, after you kill him stop using sblock and use sbar maybe?
2nd boss: you'll probably want to time an sbar to help absorb the shadow breath, otherwise use sblock mostly. Really positioning and know the fight mechanics is far more important to tanking this boss than sblock/sbar
Lei Shei (3rd boss): Sbar for the most part here. There's no real melee damage from Lei Shei herself that I'm aware of. The adds melee though but Lei Shei's damage is probably more important to mitigate.
Sha of Fear: You want to use sblock, but you want to TIME your sblocks for his flurry attack, I forget what it's called but there's a big DBM warning and it will hit you like a truck for basically 3 times in 1 global. I don't remember the exact statistics but it is possible to have sblock up for every one of these. Make sure it is, don't really worry about sbar unless you're just floating in rage.