Renegotiation is +10% crit dmg when you are using pacts when maxed.Only place i found it's worth it is fights like flame where you have intervals and short term bursts ,you can do some silly stuff with it but it's no better than the alternatives.You can ofc setup a nice pact rotation when you have no worries about mana so you dont need to keep archifiends pact for that and have some fun
As far as pets go in pve i never saw any benefit running slave,the regens from it are close next to nothing tbh.Avenger is kinda funny with some aoe in brc and in yahkmar t1 pugs to have leaders panicking it shoots fire on the boss :P.For pvp as other metioned,slave can be more annoying than other pets in pvp because of the dot it applies keeping people in combat for longer when they run away.So pve its mostly cacodemon all the way ,maybe warlord with other demos in group although buff dont stack,you can go a bit selfish with perks but yeah no really significant difference in the end.Caco is slightly more dps and otherwise you can have fun playing around with perks producing close to same results that depend more on other factors like sw and demonic payment etc etc.