Depends, if this 10% flat damage is multiplicative or additive with all other damage increase source.
If you already have something like 50% damage increase (Zei + Bane of the Powerfull), another 10% is great, but not that great. Having 20% elemental damage instead the socket will be more interesting if you're still under 100% elemental damage on your gear. However this is not taking into account the dot, ok.
It would be more easy to determine which option is the best one when we've been able to dig more about the bonus of the gems and how they are applied (multplicatively, additively, etc). Will try to perform some tests this week !
I went with Zeis, Toxin (which ticks for 3500% weapon damage ) and Bane. Moratorium was strongly attractive and I may switch yet. Teague was just punishing to keep up. Not fun gameplay. Even for a channeling build you basically start at zero every time unless you do sime SERIOUS micromanaging of familar/Magic weapon.
I for one dont really like Zei's especially for group play, it kind of forces you to hide far away from the action and furthermore enforces the boring firebird play. If you playing in a group with a monk tank I wouldn't use the Zei gem.