~110 mana vs ~170 mana. Meaning for the 'cost' of the initial direct dam on acid cloud you could drop 6 fire bombs with old Phant.
You can also directly increase the damage of phant with gear, acid gear is just more crit.
Life on Hit Returns, are 25% per mob, per tick for Phant. Acid Rain is a bit more tricky, it's higher in the center of the circle & weaker to the edge. If you get used to acid rain tho, it's alot more punchy & stronger heals.
As they are 6 & 5 sec, there's no reason you can't use both, with 3-4 other attacks between.
(Haunt another 95 weap dam per sec, locust 45, that's 235% weap dam per sec sustained with a 1 sec brake per cycle move/suck up slower weaps, add ontop fire dogs x4 [45 dps], and a guarg bruser, that's 380% weap dam per sec, sustained! More effort but double the actual damage of a WW barb!!!) (Also not the most effective dot build for us, just an example)