The M4 is larger and heavier than the micro. If I'm not mistaken the M4 has a longer battery life and uses a more common battery. Although the micro is very durable I believe the M4 is even more so. I'm a fan of the micro.
“The bitterness of poor quality Lingers long after The sweetness of low price is forgotten.”
Originally Posted by Quentin
"The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together."