This debate has taken place a million times and you will probably catch hell for asking.

That being said, this is an AR-15 based board, so most will say 5.56mm.

There are still many of us that choose the 5.45mm round though.  Let me give you the short version of the whole arguement.

The 5.56mm round does more tissue damage than the 5.45mm round due to fragmentation.  On the otherhand, the fragmentation will only happen within a certain velocity threshold. The range can be pretty good to almost worthless depending on the myriad of rounds chosin.

The 5.56mm round is almost always more accurate due to better BC and weapon platform.  The trade off is reliability.  

The 5.45mm round is effective to near its' maximum range wich will be farther than you can see and hit someone.  The damage is not as much at any range, but it reatains its ability at almost all ranges as a trade off.

The Ak platform is far more reliable than the Ar under poor conditions that are log term. While many can rightly say that the AR is reliable if kept clean, almost all can say the AK platform is almost always reliable when filthy.

It really depends on what you are looking for the round to do.  If you are looking for a SHTF caliber, 5.56 is by far more common and is the better choice.  If you live in a harsh enviroment and do not clean your weapon often, 5.45 is the better choice.

The 5.56mm round is far more adaptable with light to heavy bullets.  In 5.45 you are limited to a few bullet styles.

If you are looking at target shooting, the 5.56mm is the better choice.

This goes both ways for as far as the eye can see.

Rather than label you another pointless shit disturbing troll

If you have the right 5.56 that fragments (not all rounds are designed to act like the ones in Fackler's tests), and a long enough bbl/short enough range to hit at at least 2600-2700fps, then the 5.56 is great. Otherwise gimmie 5.45. Jack of all trades or does nothing well rants will rage but it tumbles hard, has better sectional density to penetrate without fancy materials, and if you have any doubts about ballistic efficiency then compare the shape of the 5.45 with the shooting contest 5.56 like a Nosler 77gr and see a clone.

Besides the robust AK action, the greater taper of the 5.45 helps feed/extract. The AR action and straighter cartridge allows for more consistent chamber sealing and better accuracy.

Kinda funny how the fuss over 115gr 6.8 for ARs is proving that Mr Kalashnikov was right about wanting a modernized 7.62x39 round instead of 5.45 30 years ago.