1. Personally, do you like more damage or a feeling of invincibility?
2. Where are your stat points? If you already have high life, might as well go HoW b/c you don't need more; conversely, if you went big in str/dex, you could probably use some life.
3. What merc? If you're not using a might merc, you probably need HoW for the damage. If you are using one, HoW will give you even more damage and OS will make you more rounded.
Near the top of the forums there is a handy search function that lets you search all threads in any of the forums for topics that might have been covered. Your best bet is to try there and if that doesnt help(which im sure it will since I have seen this question before), try scrolling through the first few pages of the forums to find the answer.
At the time of writing this 4 out of the first 5 non-sticky threads on this forum were about fury druids. I am sure you will have luck finding the answer this way.
I think it really depends, but for offensive builds, I think I would most likely go for HoW. It will give you the increase damage and AR that you may need. Oak Sage is helpful in a group situation since everyone gains something from it, while HoW only helps out physical combat.