I'm building an encounter in which a party of 4 PCs has recruited another adventuring party to help them fight in a single battle against a powerful enemy.
What should I consider when deciding whether the allied NPC party should have NPC monster stats (evoker, knight, etc.) or PC levels (wizard, fighter, etc.)?
I'm trying to strike a balance between two main criteria:
Balance: The NPC monsters seem to have more HP and less damage output than similarly-leveled PC classes. Would this difference in stats make a big difference in the encounter?
Ease of Running: PC-classed NPCs tend to be a lot more complicated than monster NPCs. While that creates a lot of interesting options, it also makes increases DM overhead.
I'm looking for answers that are backed up with actual table experience in this specific scenario, that can compare a battle with NPC allies vs. PC-classed allies. I am not looking for a general comparison of NPC monsters vs. PC classes, and I am not worried about the NPCs sticking around long-term.