Warlocks: Soul Harvester or Abyss Shard?

This is a very controversial subject. A lot of people say the shard because there’s nothing like it but a lot of people say the shard is useless because it’s not an instant cast and the only thing you’re saving is a little mana that you can just life tap to get back and 1 soul shard. And there’s a 30 minute cool down. Do people use this trinket for Demonic Sacrfice spec in raids? Should I go with the scythe I’ll be using for a few levels? What do you think? I’m affliction