Let me preface this by saying that
of course the details will vary based on what you fight, how you fight it, etc. ... but if you do the math, the epic seems clearly better than the Fungi.
Wall of Math inc!
Let's call an average fight two minutes, for simplicity (although it only has to last 1.5 minutes for this math to work).
Curse of the Spirits deals roughly 1425 damage in 1.5 minutes. If you recast it after the first cast finishes, you get very minimal damage for the remaining half minute, so let's just ignore that damage, and also let's round down and call it 1.4k damage for the fight.
OP said he was 51, so that means he's using
E-Bolt, which produces 3.994 damage per mana and
Plague, 4.23 DPM. Let's go with Plague as it's more efficient: if you could magically cast two Plagues at once and have the second deal the same damage as the epic, it would cost you 330 mana (1400/4.23) to get an epics' worth of damage.
Now
of course you can't cast two Plagues at once, making the epic significantly more valuable, because it stacks with your other DoTs. But still, let's compare it to the Fungi. The Fungi gives 2.5hp per second, so in a two minute fight you get 300 HP. If you turned that HP into mana with
Cannibalize II, you'd get about 0.5 mana per HP,or 150 mana (it varies by level so it's hard to give an exact number). Same deal (in reverse) if the Fungi saves you from having to cast
Superior Healing: it gives Shaman about 2hp per mana, so again the Fungi's 300 HP is worth 150 mana.
Even if you assume a two-minute down-time after every fight, that's 600 HP from the fungi, or 300 mana ... vs, the 330 mana's worth of damage from the epic. The epic may even do more damage in a longer fight, and it will do a lot more damage if you fight two or more mobs at once. But most importantly, because it stacks with other DoTs it lets you kill the mob significantly faster, meaning you take less damage (not even factored into my math).
Now to be fair, the epic costs about twice what a fungi costs, and that math shows it isn't twice as good. Also, if you're killing low level (for your level) mobs super-quickly, the Epic doesn't get to do its full damage, and the math changes.
But in general, if your options are 50k plat and a fungi, or no fungi and and an epic, the basic math shows how the epic is more valuable.