.300 Blackout is a relatively new caliber, designed to work with existing 5.56/.223 AR-15 type rifles, requiring only a larger bore barrel change. Compared to 5.56, .300 Blackout offers a heavier projectile, with more take down energy at most distances. It also comes in a sub-sonic easily suppressed round, allowing for quiet but extremely effective CQB combat.
The trade-off is .300 Blackout ammunition is about twice as expensive than 5.56/.223 ammo, and is also harder to find. If you are budget constrained, or if you are concerned about ammo availability in a post-event world, you should stick with 5.56/.223 chambered weapons. However if you want the best available ammo for CQB or you plan on doing a lot of suppressed shooting, .300 Blackout is an excellent round!