Sup. According to icyveins (I have grimoire of supremacy) Observer is better than Fel Imp. But if i put my demons on a training dummy fel imp has more dps than the Observer. In what way is the observer better than the Fel Imp? Sorry if this is a repost!
Raid buffs/debuffs put observer on top.
The dps difference between the two is actually pretty minimal. Generally observer is used for single target fights, but the imp is used for multi-target as it is quicker at switching targets.
If you are raiding you should also look at which demon will help your raid. The interrupt from observer is good if you are lacking in that, imp can be used on Thok to help healers with dispels etc.
Same thread keeps popping up all the time, can people actually learn how to use the search function?
Following things is in 9/10 cases the same deal with all
Recount + Curse of Elements.
Herald of the Titans
It doesn't matter what pet you use, their dps has been normalized, use whatever you feel like.
There really is no difference.
Have fun with it.
Each Demon's DPS is well within one standard deviation of the next. It will make more difference how you use it, and how the encounter fits it, than which one you actually choose.
Imp is good for moving bosses, because it has range so it loses less uptime.
Observer is good if interrupts are a thing.
Shivarra knock-back can also be very useful.
Use what you want or what fits with the fight, their DPS output is close enough for that.