Spore Burst notifies mobs, Mass Hypnosis does not. <-- this is probably the biggest thing. The Mind Dom can keep applying Mass Hypnosis until everyone is asleep, then people can go in and select one to fight. As soon as Spore Burst hit, any hero that didn't sleep would go gangbusters on the villains.
Spore Burst is tagged with the AOE vector, so can be avoided with some defense (not sure if any of the Phalanx have typed vs positional defense).