Unchained: Negates the damage penalty of Defiance, has a 120s CD.
Inner Beast: Heals for 100% of damage dealt and reduces damage taken by 20% for 6s.

So, ideally you want to use Unchained when you have all your offensive CDs up (Berserk + Internal Release). You can use it on trash or on bosses, depending on when you need the damage most. It is best suited for bosses unless you will be taking 2 minutes on the trash before that boss, in which case the CD will reset by the time you get there. Unchained can also be used in conjunction with Bloodbath to boost the healing you receive from your attacks. A good way to get a lot of healing on trash is to pop Vengeance, Unchained, Bloodbath, Berserk, and Internal Release (not all at once, but in between your OP spam) and use OP as many times as you can on as many mobs as you can. You will practically keep yourself topped up on your own this way.

Inner Beast, meanwhile, is used for mitigation purposes. If you don't have Unchained available, IB is your go-to button for damage on single target enemies and to mitigate damage. @50 you'll be using this to mitigate big hits and to mitigate small incoming damage/heal/do damage yourself. Pre-50, however, there aren't any real "big" hits, so you'll be using IB whenever it's available to you.

@45 you'll get access to Steel Cyclone which is essentially a stronger Overpower that costs you 0 TP, only your 5 Wrath stacks. This is used on trash when you have other mitigation CDs available (Vengeance, Thrill of Battle, etc.) and want to get aggro/do damage instead of using Inner Beast.

That's the jist of it.