Ummm, in my case I tried both but finally I've choosed to prank Forharn.
The reason why are the goods you can loot from the house of Rognar the Hunter (on Slums of Thule) using the key Arneta will give you. Well, strictly speaking, what worth is mainly one item, a very valuable and highly enchantable ring (by far, the best I've found on the game).
I was also quite reluctant to finish the Innkeeper's Brother Quest, as you loose the Innkeeper's ring (Armor +10), but the new ring is even better. Now, I can finish the quest, get the Dagger of Aflicttion... and use this one on Thule's Abbey...