They are both good Pokemon.
Spiritomb is, as you said, a tank-y Pokemon. Its low health and speed, as well as Poison being its only resistance, means it will need support even though it has no weaknesses.
Mismagius has the Psychic and Dark weaknesses, like all Ghosts, but it has a great 105 in Special Attack, Special Defense, and Speed.
Health is slightly better than Spiritomb's, but Defense and Attack plummet. But, as a Ghost-type, technically Mismagius shouldn't really need to rely on either too heavily.
Magnezone = Special Sweeper
Blastoise = Physical Tank
Clefable = Totally depends on the set you're running
Zoroark = An incredibly tricky Sweeper, probably Physical, can run Special
Heracross = Physical Sweeper
You don't have as much tanky stuff as you think. You're running 3, potentially 4 Sweepers with Clefable.