I like both Loki and Emma and have them both at 60 with full uniques.
Emma really has only one viable build, Omega Telepath+Hellfire Mistress hybrid. I've never seen anyone play her as pure Diamond form. That said, both Emma and Loki deal huge amounts of aoe and dot damage. They are very similar in a lot of ways, except that you can NOT currently play Emma as a summoner because her mind controlled minion is pitiful even after investing in summoner gear.
All of Loki's trees are good. I hear the summoner build is effective (though I'm not a fan of it), and the other trees give you good options for dots and aoe. Loki also has amazing survivability (not so with Emma), because he has a no cooldown teleport that also makes him invisible and summons an image (extremely valuable in this kind of game, obviously), and his summons take a lot of the aggro. Loki is much easier to play because you will die a lot less and you can port around very quickly. All Emma has for travel is a dash. Loki should also be a much better farmer than Emma, and his synergy is better than hers.
I think test center is up, so I suggest getting on it and trying them both for free. But overall, Loki has more benefits.