You're wrong about the Druids; if you're leveling up a hero with Summoning or Chaos skills, it's a complete waste. Try it out. Hmm, I don't really know – I choose based on my mood. Both upgrades are usually viable; it's a matter of taste, as well as overall tactics and the expected enemy.
And regarding the new Dark Elf creatures, I don't even know; it depends on who you're fighting. The old ones are more than good enough for reducing enemy defense.