This is a debate that I have with myself almost daily regarding my paladin. I have also asked my guildie holy paladins about this several times.
I think chip's post is pretty accurate.
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If your focus is PvP, then Holy Shock is most definitely the way to go. PvP dmg is much burstier than PvE, and often times you need a quick tool to keep your target alive before the 2.5 secs of Holy Light hit him. By the same token, hitting a retreating warrior so he can't drop combat-charge, or finishing a low health runner are stuff that can often times mean a significant advantage.
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The other thing with Holy Shock is that it's a pretty good tool if you want to do the eclectic combination of raid healing and dungeon tanking. Let's assume this build:
http://db.vanillagaming.org/?talent#sVxubgktVRtIOne of the perks paladins as a class is getting the bulk of their tanking talents in the very first 3 tiers of protection. (Imp Hoj/Imp Conc in the fourth tier aren't necessary for tanking, reckoning is more pvp oriented, and 1H Weapon spec only affects physical damage). With enough spell damage/healing, Shock can do a lot for you tanking. A dmg spike suddenly hits you? Cast it on yourself to help your healer, get the heal threat (which is 0.5 ratio in this server) on top of surviving. Want to pull yourself, and the mob isn't undead/demon? Shock it and enjoy a very decent starter aggro. One of your ranged DPS got aggro? Shock again and it'll most times get it back to you. It can be a pseudo-taunt, akin to Shaman Earth Shock.
I had a thread on this same issue months ago:
https://forum.nostalrius.org/viewtopic.php?f=43&t=26018The feedback from other paladins was:
Alexandros wrote:honestly when tanking holy shock is the best tanking talent you can have since it works as a semi taunt with divine favor and ranged pull so u dont need to carry nades.
and get imp righteous fury 3/3, you will thank me later
JCarrill0 wrote:Agreed, sub taunt ability makes things useful for dungeon tanking
Another issue to consider is: Holy Shock allows you to do emergency tanking in a way BoSanc wont.
So let's say you got hard CCed while healing (long stun/sleep/mind control), your tank took a couple of nasty hits and he died before you could get back on your feet. If you have a priest, druid or another paladin dpsing, there is a room for an amazing play that I've managed to pull succesfully twice so far: Toggle Righteous Fury on, SoR up, Shock + Judgement the mob, this will put them on you just like if you had taunted it, and have your offhealer keep you up. It will not last long since they might not be geared for it or maybe they were low on mana while dpsing, but it can certainly afford some 20-30 seconds to finish up the mobs and prevent a total wipe. Of course this requires you to have heal mail/plate and a shield rather than a nancyboy dress and a Lei of the Lifegiver cute little rose to keep up (but at least this one you can swap in combat I guess).
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Now, regarding healing 5 mans: Practically every single Holy Paladin I talk to tells me Holy Shock is used rarely since the heal/mana efficiency isn't too good, to ever prefer it over FoL spam, and it only really comes to use when a pull went wrong and a fast emergency heal is needed. On the contrary, putting BoSanc on your warrior tank is actually something that will help for the full duration of the run. Not only because of the dmg reduction making it easier to downrank FoL and keep up fine with damage, but also because a good amount of warrior tanks aren't good at managing multiple targets. I can keep 6, 7 targets on me just fine when I tank with a warrior, yet while healing I've come to know guys that can at best manage 2 or 3. Putting BoSanc on them gives them reflect dmg on shield block. Compound that with retribution aura and suddenly they don't suck as much and have a shot at holding aggro. I guarantee you any prot warrior will be pleasantly surprised to have BoSanc in a dungeon.
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Regarding raid healing: You only need 1 BoSanc Paladin to keep the 15 min buff up on the warriors, so unless you are the designated guy for it, having BoSanc ends up being redundant and a waste. Shock is then the way to go. It allows you for a slight amount of multitarget healing (You have FoL/HL on your primary heal target/tank, but some dude just took a heavy AoE, swap targets for a second, shock him, and get back to business), useful vs spikes on tanks, and to build on chip's meter point: It makes your HPS look better for when the epixx lewt is being distributed among healers.
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Summing up:
If you will do PvP Healing: Holy Shock.
If you want to tank dungeons, UBRS or maybe OT 20 mans for the challenge part of it: Holy Shock.
If you will only do PvE healing and you can afford to be designated guild guy for it: BoSanc.
Makes you stand out from the other hpaladins and earns you more tank love (and by tank I mean warrior, since the buff is wasted on druids).