I have 3 surplus packs and I'll give my comparison of them.
Medium ALICE pack - Cheap as dirt, but durable and plenty of mods available (see Wildcat Mod). I made mine more comfortable by slicing up the kidney pad to use the belt and connectors and threading it through a Condor padded MOLLE belt, which I inverted and attached via the D-rings to give waist support (as the kidney pad rests above your waistline and doesn't transfer weight to hips) and I found a set of Blackhawk Enhanced ALICE straps for more cushion. I even rigged my FILBE frame and straps to work with it and it gives lots of room below the pack. I used it when I was in the US Marine Corps without those mods and kept it around. It is rarely used now as I usually us my USMC Pack Assault Pack (FILBE), but when I want to look old-school, I whip it out. Makes a great bug-out-bag. I had the Large ALICE, but gave it to my brother as it just sticks out too far and makes loads too unwieldy.
USMC ILBE Pack - Got a decent deal on an EXCELLENT Condition (Grade 1) used set, make sure if you buy one, it has lid, waist belt and radio pouch. The lid doubles as a waist pack (haven't used it in this manner, but the straps are hidden for this). Internal frame, alpine style, very comfortable and transfers weight to hips. This thing weighs much more than the civilian Arcteryx due to heavier materials, a lot of PALS/MOLLE webbing and straps. Side pass-through pockets which can carry mortar rounds or other long items, but can get tight if you stuffed your pack with too much gear. Even though it is heavy, it's comfortable and will probably last much longer than I can use it. It was mainly dropped from use because it doesn't fit well with body armor and the fact that it is tall enough behind your head that if wearing in a prone position, you can't move your head when wearing a helmet. I don't wear body armor, but it does make contact with my pith helmet even when hiking/walking. It also gets a little warm on my back. Only comes in Woodland Marpat camo, which, some people don't care for. It is just sitting in storage right now just in case I want to use it.
USMC Pack (FILBE), complete set - My favorite of all military packs that I have owned or tried. Heavy son-of-a-bitch, but will last forever. If you make this purchase, check the lot number to see if you need the Oneiros Valley USMC Pack Modification Kit. Mine did and I used it to strengthen the shoulder straps and even bought a new 1606MC (second gen.) frame for it. You don't really need this because you probably won't be dropping it from the back of a 5-ton truck bed with a full load or bending the frame like you may have seen in some youtube videos, but I love buying stuff that I don't need. This pack is versatile and you can have almost endless combinations of pouches on it, but it's big enough that you usually wouldn't need any. I put the two sustainment pouches on with the two bladder pouches because I carry all of my gear and my seven year old son's gear as well. It's easy to over pack, but the compression straps allow you to get down to a low profile. I used the main pack on an Alaskan Moose hunt (successful, but another story) and carried all my supplies for a 2 week hunt in the wilderness. I find myself taking the Assault Pack most of the time, as I don't do too many long-range hikes and I use at least one sustainment pouch with it to carry my cooking stove and food. Main pack has larger pass-through pockets which don't tighten up with fully stuffed main compartment (unlike the ILBE), great for slipping in my Ray Mears Wilderness Axe and saw kit or my rifle or any other long items (it's made for mortar rounds). The frame, harness and waist belt are comfortable, but I have been trying to get a Mystery Ranch NICE frame for it to make it even more comfortable, but at the prices that those typically sell for, I might as well pay for a Stone Glacier, Kifaru or Mystery Ranch pack. This thing looks like it will last forever with civilian use but I am sure that as I age, I will want something lighter, but I am not willing to shell out the cash for the more expensive brands out there yet. Yeah, buy once, cry once, but dealing with the wife would be crying multiple times, right? I suggest the full kit, but you can get all the parts separately for whatever your needs are and at fairly low prices. Yes, you pay shipping from the U.S., but still a bargain from the prices that I see posted on .uk sites for the same pack.
If anyone has questions about any of these packs, feel free to post or pm me directly.