The military has adopted so many variations of load bearing gear that it can be mind boggling. I will attempt to break down each type concisely and briefly here for you. I have extensive experience with the ALICE, ILBE, and FILBE as a Marine, however, I do not have as much experience with the MOLLE II platform.
ALICE- not very modular but very durable with a metal external frame. There are at least three sizes of ALICE gear that I am aware of and several "boutique" makers as well. Generally, the Medium ALICE was issued to support troops like tankers and such that found space to be at a premium and did not need to physically carry as much. The large ALICE or Mountain Ruck as it was known in the Marines could carry enough weight to sustain a Marine for at least a week of combat operations. Both packs look very similar and were often modified by the owners to better suit their job needs. Snipers and Recon was notorious for this. ALICE can be found on Olive Drab and Woodland Camo among other patterns.
ILBE- This is a USMC surplus pack designed by Arc'teryx and is an internal frame design and is a tall/skinny pack that rides very close to the body. This pack was also designed with a smaller assault pack that could be used in conjunction with the main pack or as a stand-alone 3 day pack. This pack is very well liked amongst civilian circles but created many issues with hard body armor wearing users. It was not designed to interface with the issued armor that was issued at the same time. If you are hiking slick, this is a very valid option even given its age. The chief complaints are mainly weight and difficulty repairing it (requires a commercial machine). we are working on developing a couple weight saving options for people to shed nearly half a pound from its weight while increasing its durability and water resistance.
FILBE- This is the current issue pack in the Marines and it was designed by Mystery Ranch. It is an external frame design utilizing a plastic frame. It is very modular and when a part breaks, it is easy to replace that one part instead of having to get rid of the whole ruck. It interfaces with body armor much better than the previous ILBE and is rated for loads up to 200lbs. This pack had some early teething problems with the first generation of frame cracking in the cold, however, these issues were largely fixed with the second gen frame.
MOLLE II- These packs have been in service for decades and are very average in virtually all regards. They have been issued in ACU and Multi-cam/OCP. They are a workhorse and have served millions of service-members over many, many combat deployments. There are no secrets with this plastic external framed pack.