Let's talk about which one is the most powerful. Also, no saying " Dremora Lord is the best " only these two.
I like Atromancy more I think, because it doesn't require a body you just cast it, you can cast it anywhere, before the fight, they can use both melee and ranged attacks and they look cool.
Necromancy is powerful after Dead Thrall but I don't like it, sure I like the concept and the coolness but still it requires a body, right body.
Let's discuss!
I'm very much a conjuration mage, and I have never once used Necromancy in 528 hours of gaming. Never will, either. It's just horrible, for a start - my character (and his predecessor) would never use such debased magics. Luckily I love the elemental Atronachs - well, two of them. I love the Flame Atronach because she has good range, which can be useful in clearing rooms with high walkways or in getting the attention of flying dragons; also, I think she is one of the most beautiful things I've ever seen in a game. I'm also fond of the Frost Atronach, partly because I largely play as a frost mage but also because I find him very useful in caverns and dungeons - conjure him in front of you, and no-one can get past him in tight corridors to get at you. And I enjoy his big lumbering footsteps behind me when travelling, too. I've never really taken to the Storm Atronach - I remember being disappointed when I first saw him (Storm Atronach? He's made of rocks!) and I have never really got over that. He has his uses.
Also, I love my insanely aggressive Dremora Lords. I'm saying that, and you can't stop me.
Necromancy all the way!!!
Atronachs are based off elements and are weaker to one of them but amuse to the opposite one.
Necromancy on the other hand is equal to each element and doesn't have a weakness.
Another pro of necromancy is that you can raise any body so that is much better than summoning an element.
Finally, in your first comment u say that there is no range with undead. This is false as u can raise a dead archer or (for those great necromancers) a summoning from soul cairn... this is actually not Atromancy.
Necromancy. The spells are cheaper to cast, provide a temporary storage chest and, with the Soul Cairn Summons, have range and melee fighters that can be healed with necromantic healing/heal undead
Atronachs are too bulky to fit in tight spaces (least the frost and storm atronachs are) and have a terrible AI which, more often than not, results in them not holding aggro with the frost atronach being the the only exception.
The new summons in the Soul Carin are sweet. Before, I would only use Atronmancy because it's predicable and you know what you're getting every time you use it. When you raise some dead with Necromancy, sometimes you raise a bandit who will just use their fists and others who use their weapon.
Sure Soul Cairn summons are sweet but I just made my Storm Atronach and Wrathman fight each other. Sure Wrathman is owerful but still Storm Atronach won. Because when its is low on health it starts fleeing into a safe distance regaining some health and use ranged spells and repeat it until it has enough health then goes into melee does a lot of stuns. But the winner was Storm Atronach.
I was actually going to name this thread 'Summons or Necromancy' but Atromancy seemed a bit cooler.