On p99 ...
Arch lich is 20 mana for 20 damage (1:1 ratio) - Confirmed.
Demi Lich is 31 mana for 48 health (worse ratio).
If arch lich were 31/20 as
http://wiki.project1999.com/Arch_Lich would suggest --- that makes the 49 lich spel (22:20)l the most efficient. Thankfully, Arch Lich's saving grace is it is in fact 20:20 ... which gives it a use.
Depending on buffs and what you are doing Demi is undeniably better. If you are iksar and have regrowth ... go Demi. When solo and having to use mana to heal yourself back - Arch lich is better despite being 11 less mana a tick (costs you 28 health a tick).
It depends on what you are doing. In challenging content solo or with a group - demi lich give more mana thus yields more through-put. You have to heal back the extra damage, but those heals do damage to your target even if the difference in mana regen is not offset by heals. For more mundane content, arch lich is better imo.