The age old question: ILBE or Molle II?
Personal preference is basically what it comes down to but most people don't want to buy both just for test runs. I was in the Army, not the Marines so my issued pack was the Molle II, but I have used the ILBE as well, and there are pros and cons to both.
First off, let me say that I am partial to the Molle II but it is mainly because it was my issued gear, and it is what I became most comfortable with.
I felt like the setup for gear storage on the Molle II is a little better (again personal preference, someone who liked the ILBE may would argue the opposite). The sustainment pouches on the sides are awesome for storing the stuff you really don't want to dig for like rain gear, tent, water, etc. There is also a zippered bottom and the ability to create two separate compartments in the Molle II. the bottom is usually used for your MSS (I highly recommend one of those too. Great sleep system, but a little heavy). Mine was ACU. That sticks out like a sore thumb no matter what environment you're in -so definitely go with MultiCam. The thing I didn't care for too much was that it was incredibly easy to make the Molle II, top heavy. I often felt like I was hunched over at the waist to keep the load balanced (Disclaimer here: I carried almost my entire set of deployment gear in it, which included all my body armor, mags and ammo. I weighed the pack once at a little over 121lbs - so very durable construction). This was never an issue with lighter loads. The 3-day assault pack is an integrated part of the Molle II and can be attached via straps and clips, which was always a really nice feature. The Molle II does have a plastic frame but most come with the "upgraded" frame and they hold up fine. Mine never broke, nor did any of my battle's packs.
The ILBE has one main advantage in my opinion over the Molle II: Load Balance. The ILBE seemed to distribute weight a little more evenly across the body. This is likely due to the more narrow verticle outline. The ILBE also has an integrated 3 day assault pack that attaches just like the Molle II system. The ILBE has the same two compartment setup and has access to the bottom from side zippers (the Molle has front access via a zippered pouch) I think the MultiCam on the Molle II is a little better than the digital pattern on the ILBE, but again - that's my personal preference. The ILBE is smaller cubic inch wise than the Molle II but is more than adequate in size for a BOB. The frame/strap system for the ILBE is constructed by Arcterx which is a plus. There is no designated area for a sleep system on the ILBE but it can be attached ith straps or you can make room in the pack itself.
I think either way you are making a good choice. Both are adaptable, reliable, and spacious. From all the comparisons I have looked at, it seems that, to the best I can figure, people prefer the ILBE in the neighborhood of 60-40 compared to the Molle II. That's completely subjective though.
I hope this helps some. These are kind of the broad highlights of my experiences with both packs.
Here is a link to a YouTube comparing the two (I have not watched it as of yet because having tried both I never really needed to look, but I will now):