Which would you personally recommend for a Demonology PvE Warlock - Grimoire of Supremacy or Grimoire of Service?
Really hard to say... at least at 85.
Based on my tests with it... and keep in mind it is also dependant on HOW you play... it is nearly even between using Supremacy with Wrathguard and using Service : Felguard. I think Service MAY come out ahead if you can stack many DPS CDs on top of each other or is useful if you have a boss fight involving short term buffs. As for myself... EVEN though I do play a very CD dependant Demonology warlock style and in consideration of that Service MAY come out ahead as a result... I like the looks of the pets more I end up using Supremacy. That 20% bonus damage claim of supremacy doesn't quite prove to be true though... and because of this I feel supremacy could stand a small buff as a result. I can tell you what needs to be reworked though... and that is those awful level 90 warlock talents. With the POSSIBLE exception of the AOE talent... the other two are just awful or incredibly situational.
Back on topic though... I would recommend Supremacy for most demonology warlocks just because it's less maintenance. Demonology Warlocks already have way too much to worry about from what I have seen... even though I can handle it well.