For demo it's dependent on your trinket selection. If you are using "on use" trinkets Service is better than Supremacy since you can roll all those buffs together into that second pet. If you are using proc trinkets Supremacy will be better since you can't line the procs up well with Service. If you were to look at the meters of each talent choice Service would be very up and down due to the on use spikes. Supremacy is more consistent but doesn't spike nearly as high.
Basically worth it for every boss fight assuming you dont have the current 4-set. It gets a lot more complicated once you obtain the 4-set, because you can no longer synch it up with dark soul, and suddenly the choice between the two talents becomes encounter dependant.