AFAIK, Venomous Spirit is somewhat homing, but about as slow and deals much less damage. Punisher doesn't home, but fires three projectiles at once for extremely high poison damage to anything they manage to hit. In my admittedly limited experience, Punisher will flat-out kill you even with 80%+ poison res, while VS deals largely negligeble damage even with ~50% poison res. So, Punisher is much better as long as you're able to reliably hit things, while VS is mostly useless as long as you have any other sources of poison damage.
Anyway, do higher difficulties actually spawn larger numbers of enemies, or am I just imagining things? And do mercs gain more XP than the PC as long as they're lower level, or is the appearance of such just that they need less to level up?