Venom Hydra is more advantageous in almost every situation.
While the pools of acid do less damage per second, they will stack on top of each other, and with 3 heads as opposed to 1, this can mean that it's actually doing 54% damage per second in one location (which can hit multiple mobs), or 18% in 3 different locations.
Venom Hydra also has speed in terms of the fact that Mammoth Hydra's flaming ground is fairly slow to reach it's target.
Venom Hydra is the clear cut winner in situations where an enemy will stand still for extended periods of time (like bosses or if you're grouping with a melee character), as the dps output on targets standing in pools is just insane.
The only time that Mammoth Hydra seems to win out is if lots of enemies are queuing up (say coming through a doorway), but even then, Venom Hydra can hit the first enemy, and when it dies, the second enemy will take it's place which in turn will mean it's now standing on the venom pools. So Mammoth Hydra is only barely better in this situation, while Venom Hydra is far better in all other situations.