Both are important but Lifesteal is better the higher your dps output is, if your looking for sustain at higher MP levels a mix of both is good,
I currently have 6% Life steal and 893 Life on Hit with 664 regen per second and I farm MP5/6 and can do MP8 but its way to slow, I may have to much but I never die.
Have close to 1300 LoH, and can easily face tank poison&molten on MP10 (even with both affixes togheter), but Damage Reflect Horde has become a real problem if I can't kill these elites in about 10 seconds (few seconds before serenity, serenity and few seconds after serenity).
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