Hello Snarky.
Honestly, IMO depends on how much control you like. Lich has some good control at the expense of some damage vs. Demon Prince. Undead also are immune to slows, but are not quite as fast as demons in the ground obstacle course; demons block doors and are noisy, undead are silent. Both perform well, especially with the update to Necromancy, and I'd place them about on par with each other whereas previously undead were behind demons by quite a bit. No surprises there.
Demons is just great for damage, but when I feel like having some control (more than Demons, as it is), I go with Necromancy, as Lich is pretty well scripted AI for sitting in the back lines and tossing out his controls.
I have both Necro/Dark and Demon/Dark for years, and I love them both so much, I can't call either better than the other, especially now.
Just go with theme, as both are solid performers, neither disappoints.
TLDR: both are great. Undead over Demons only if you like a small tradeoff between damage for control.