It's hard to judge at time. If you do some napkin math and assuming nothing overheals:
Chain Heal = 380% + 323% + 274.6% + 233.4% + 198.4% = 1409% spellpower
1409% Spellpower * 1.10 (floodwaters) = 1550% spellpower
HST = 82% spellpower per tick * 1.30 (Floodwater) + 72% (Queen's Decree) = 178.6% spellpower per tick
178.6% * 7.5 ticks = 1340% spellpower
HST costs 24.200 mana
CH costs 55.000 mana
HPS is really unable to be judged as HST does very little HPS, whereas CH does a ton.
Assuming you can get 1 CH cast of in the span of 1 HST tick, you're looking at CH doing 8 times more HPS.
That is why you generally use HPCT, which heal is the most efficient for the time you spend casting it.
HST costs 1 GCD (1.5 seconds); whereas chain heal costs 2.5 seconds to cast. (Haste affects both equally up to 50%, so im just striping that off).
That makes HST do 893% SP/cast time and CH do 620% SP/cast time
When you look at HPM.
HST heals 0.0554% spellpower per mana; or in other words 18.06 mana per spellpower
CH heals 0.0282% spellpower per mana; or in other words 35.48 mana per spellpower
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Looking at logs you'll likely see quite large discrepancies though. Mostly because Queen's Decree can't crit and most of it's healing is overhealing so it's value is largely reduced.
Looking at a few logs I would say that Chain Heal does about 1.5x more healing per cast than HST on average.
For HPCT that means that HST is still better as it costs less time to cast, the gap is much smaller though. HST is about 10% better HPCT than CH.
For HPM HST is also still far superior.
So for maximum healing output casting HST over CH is always worth it, unless you happen to have CBT or AG up.
Jynus might be right that CH does more healing per cast. But it's still worth putting down extra HST, because the time you waste putting them down (1 GCD) is less than the time you use casting Chain Heal and if you correct for that time spend HST does more healing (= higher HPCT).
I'm not sure where you get the idea that you have extra GCDs; I think you're misunderstanding TigerTiddles in that post. He's comparing 3-4 CH to 6 HST not based on healing done, but based on the fact 4 CH costs about the same time to cast as it costs you to cast down 6 HST. You dont get ANY EXTRA GCDs as you have "wasted" them on the longer cast time of chain heal.