Re
I have played a necro to 40 and a mage to 30 at the current point in time. Plus played a necro to 65 and mage to 53 on live. (yes i know its different)
Basically:
Necromancer
-----------------
Very good soloer
Insane mana regen
Best DoT damage in the game
Feign Death
Can Feed Mana or Health to others
Lifetaps (DoTs and DD for heals)
Pets (Heals, Haste)
Snare DoTs
Undead based spells (slow, mez, root, DD, etc)
DDs are poison based
Magician
-----------------
Good soloer
Mana regen based on items
Summons Items (Food, Drink, Weapons, Bags, etc)
Gives Mana thru Mod Rods
No Self Heal besides bandages
4 Pet choices (all with different abilities)
Pet Spells (Haste, Heal, SoW)
Fire and Magic Based Nukes
Elemental Based Spells (DDs)
Debuff line of spells (Malise)
Damage Shield spells
Solo comparison
-------------------
Necro can FD in trouble
Necro better kiter since it has snare
Mage can keep mobs from running easier with Earth Pet
Mage Pets can do spell damage, stun, or root (can be better for casters with stun)
Necro can use stacked DoTs to keep aggro off the pet
Mage has better DDs (mana ratio wise (i think))
Honestly, I would probably call this a toss up. Big thing to turn me towards a necro would be crazy mana regen, but I believe that mages solo better indoors. Obviously necros have a way out with FD though if the pet dies.
Group comparison (who to choose)
-------------------------------------------
Mage offers DPS mainly. Mage can also provide summoned items like Mod Rods or FT item in later levels. Mages also have a debuff line of spells (Malise) and damage shield line of spells.
Necro offers DPS also, but a lot of their DPS comes from DoTs. Stacking DoTs in a group will bring aggro to the necro. Necros can pass mana to others, but the ratio of mana taken vs given isn't wonderful.
Overall I believe that Necros are the master of DoT damage, but in a group stacking dots and stealing aggro might not be the best thing. However they do regen mana faster, so I dunno about their DD dmg/mana ratio. In a group its pretty much a toss up again, but if you want pure DPS, with debuffs and DS...then go mage. If you want pure DPS with mana feeds and possible aggro stealing...go necro. Honestly I think the necro will outdamage the mage if he is allowed to stack dots and steal aggro (never tested this btw).