Oh boy.
Magma ticks more often and deals more damage. At about 4000 spellpower, because Searing Totem gains more damage from your spellpower than Magma does, Searing Totem will theoretically produce more damage--but it doesn't, because it ticks slower. It's not
supposed to tick slower, but it does. Its cast time can be affected by your latency, the latency of the instance, the current positioning of the boss, and the future positioning of the boss. Magma, however, always pulses right on time.
This is
one of our totem bugs that Blizzard has yet to address.
The global cooldown usage doesn't matter. There is no better damage per second ability that you can use, except an out-of-mana Fire Elemental.
Now, sure, you can claim all you want that Magma is difficult to use on bosses that move, or is difficult to refresh. But that isn't going to change the fact that it produces more damage than Searing. I theorize that Searing will become better than Magma at a currently unobtainable level of spellpower, unless it is fixed. --And Blizzard hasn't even admitted that there is a problem, so it won't be.
And another comment: I know that Searing Totem looks like it's supposed to be our single target DPS totem, and that Magma looks like it's supposed to be our AOE totem, and that Chain Lightning looks like it's supposed to be our cleave, but.. none of the above is true.
Ghostcrawler defended the fact that Chain Lightning, until certain levels of haste, will grant more DPS than Lightning Bolt.
They don't know what they're doing; you can't really trust their "design". (If you call throwing bacon at a wall and seeing what sticks design.) What works is what works. Use Searing Totem only on bosses that will move, using Magma Totem for single target and AOE, use Chain Lightning for single target until 1000/1200 haste and AOE, and use Fire Elemental on any single target boss fight--never on AOE.
By the way, if you're dropping fire damage totems on Festergut and you aren't standing in melee (unless you've reached the melee cap), you are sabotaging your own DPS on a DPS race.