http://wiki.project1999.org/Magelo_Blue:Erumancer
For kicks I made a Magelo profile demonstrating the degree to which an Erudite Necro can get away with ignoring +INT items. It's pretty impressive. Fully 50% of his slots contain no +INT items at all and he's still sitting at 204 INT. The other half of his slots can be fully devoted to +HP and mana gear. For reference, my Iksar Necro is 54 and similarly geared and has approximately 650 less mana and roughly the same HP.
The Erudite's superior natural INT combined with the non-Iksar items he can use amounts to basically a 50 point INT advantage over the Iksar when both are wearing full HP gear. This obviously gives the Erudite a much larger mana pool, which partially negates the Iksar regen since the Erudite can cast more lifetaps from full mana. The difference is almost enough to allow the Erudite two extra casts of Bond of Death, which would heal 1440 for 720 mana.
I'll warn that the rest of this post is math-heavy and is aimed at Necro enthusiasts and EQ data nerds. Pay attention and don't let your eyes glaze over!
At level 54, every minute while liched the Iksar loses 90 health in exchange for 400 mana, while the Erudite loses 160 health in exchange for 400 mana. So the Iksar efficiency rating is 4.4 mana per hp, while the Erudite's is 2.5 mana per hp. The Erudite must cast Bond every 5 minutes, while the Necro can get away with casting it every 9 minutes. This means the Erudite needs 12 Bonds per hour while the Iksar needs roughly 7.
So the difference every hour is five casts of Bond the Erudite has to make that the Iksar won't. Five casts of Bond costs 1800 mana.
So essentially, starting at t=0, the Erudite has +700 mana over the similarly geared Iksar. But at t=60, the Erudite is at -1100 mana compared to the Iksar due to the necessity of casting more taps. This means that for any duration under approximately 20-25 minutes, the Erudite is more efficient due to the big mana pool. For anything over that, the Iksar regen makes them more efficient. Very interesting.
Although to keep things in perspective, the math shows in the end the Iksar regen only amounts to an advantage that reduces med time by 3 minutes per hour compared to the Erudite (1100 mana advantage per hour for the Iksar, 3 minutes of liched medding = 1200 mana). Necros gonna necro, mana regen being sort of their thing and all.
But this isn't the whole story. Due to the XP penalty, the Iksar will need to kill 20% more mobs per hour to keep up with the Erudite's leveling speed. Let's say the Erudite is able to kill 15 mobs per hour. This means the Iksar would need to kill 18 mobs per hour to stay even. Does a +1100 mana advantage per hour allow you to kill 3 extra mobs at level 54? And can you kill those 3 mobs in less than the 3 minutes it would take the Erudite to med up and offset the per-hour mana penalty? Probably not.
So ultimately the math tells us that the Iksar is indeed the more efficient Necro race for any time over 25ish minutes. However, despite this advantage they will level slower than the Erudite, because their efficiency increases are not sufficient to offset the XP penalty. It roughly works out to a 5% efficiency advantage for Iksars, at the cost of a 20% XP penalty (Erudite needs an extra 3 min of med time per level to offset Iksar regen advantage; 3/60 = 5%).
At the end of the day, the Iksar will be a more efficient camp holder/farmer due to their regen, while the Erudite will be a faster leveler, superior PvPer, better raider (bigger mana battery) and better at breaking difficult camps and killing tough mobs solo due to their noticeably larger mana pool. It's just a matter of which you prefer.
I would be grateful if anyone could point out any mathematical errors or erroneous assumptions I've made here.