Depends.
How villainous are you?
If you want to fuck 10 people over after gaining their trust, pick the Ebony Blade.
If you just want to smash, pick Volendrung.
Unfortunately I don't have much experience with either, due to the fact that I drop the item for a second and then forget where it is XD
volendrung because it has bigger damage but when i want to murder somebody then i pick the ebony blade because it simply feels good, feels right :D i like both but the ebony blade looks so neat and elegant
I just killed all the characters in the city after helping them.. I had to reload :x
Then.. I killed all the beggars after paying them one gold.
EDIT: What do you mean with 'or' ..? Which one to use? Since you can get both you can just pick what you think is the most useful (or hotkey it from favorites)
Personally, it all depends on your playstyle since they are both excellent weapons. If you are going for a very bulkey playstyle, go with the Volendrung. This job becomes even easier with the stamina steal enchantment. On the other hand, if you are going for a lighter two-handed style, the ebony blade is the perfect choice for you. The life steel enchantment on this thing is god-like, and considering it does not need soul jems, you can keep killing with no hesitation. The thing is, you can aquire the Volendrung much earlier than the Ebony Blade. The thing is with the Volendrung, if you equip the Rahgot fragon mask and the ring you get from that cannibal quest with the Volenrung, you become a stamina machine and for that reason, I am using the Volendrung. Sorry I am wrighting this in 2019 :)