How useful is the Tremor Totem vs. Grounding Totem?
Tremor is especially useful for breaking fear attacks when you are fighting with an opponent that can fear you (a Warlock for instance). Since fear can be the most devastating weapon of an enemy like Warlocks, breaking the fear effect can be devastating for the Warlock. Tremor also dispels Charm and Sleep effects, which is especially useful when your group is heavily under attack and some of your party members fallen prey to charm and sleep.
Grounding Totem will not remove any existing debuffs cast on you, its main purpose is the take all the debuffs and negative spells cast on you to itself. Bad thing is that Grounding totem randomly chooses which spell it's going to redirect. Grounding totem is pretty useful against the casters that rely heavily on their curse and DoT spells. It's pretty useful against a mage too, although the totem will be destroyed with the first mage spell that it'll redirect to itself, because the totem has only 5 HP, no matter the rank. The totem is no longer immune to non direct damage spells so *any* spell redirected to the totem will destroy it. This includes DoTs for instance.
When do you use Tremor Totem vs Grounding Totem? In what kind of situation(s)?
Tremor is an interrupt. You can use it to stop a negative status that is already in effect - Grounding totem will not do this. A good example of Tremor's unusefulness comes when fighting something like Eranikus (Sunken Temple). He has a nasty habit of repeatedly sleeping party members, especially the MT - for this fight you can start with a Grounding totem down, then you think you'd toss Tremors as needed to interrupt any sleep effects that happen while Grounding is on cooldown, but unfortinutely Tremor Totem has no effect on Eranikus' sleep spell.
Remember, Grounding totem has 15s cooldown, Tremor has none.