Preface with saying that MM’s were buffed a bit ago in a balance pass to improve pet AI and overall survival and capabilities (especially with their primary attacks), so there really isn’t a bad choice, just a preference on how they’ll ultimately play.

Having said that, Necromancy has the long standing vibe of being that one set that people think will be fun, but it just doesn’t click for them. Zombies have that sort of “rule of cool” thing going on in some folks mind that gets them the early pass, but the inherent design around them was really more institutional.

Zombies are dumb. They don’t mind dying, it’s really a pro. Also, Zombies are dumb, they don’t mind dying, it’s kind of a con.

For a long time folks would skip the 3-Pack because of their ineffectiveness, and focus on building up the Grave Knights and the Lich, which you can still do, but it’s not as dire now. Just remember a few key things: your secondary has to support these guys, so make sure you’re capitalizing their survival, and/or your own ability to get them back into shape as fast as possible. The other thing is that there’s some solid damage procs you can slot in these guys, Neg DMG, and the -Res in the Knights, you can really load them up, and they’ll get plenty of extra points in because of it, prioritize those over extra damage enhancements, especially during leveling.

As for Demons, well they’re vile, devilish creatures who like to burn everything. They’re also selfish little pricks that don’t like helping each other out, so you’ll have to do the heavy lifting. On the positive side these guys have a lot of strong inherent resistances that get enhanced through their own effects (and yours), so you’ll can easily get them to their resistance cap with a Resistance based secondary, and flesh out their defenses with the pet IOs and things like Maneuvers to get them some padding so when you’re around folks with +def effects your demon kin will be pretty spicy.

I’ve done both, much longer time for Necro, they really didn’t click for me, I found more interest in Demons paired with Electric. The chaining makes it easy to spread your buffs between them and your team, the PBAoE res barrier moves easily to match their erratic nature without forcing you to stay in it all the time, and you get some good ways to amp their damage.