depends on several things IMO:
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.