I have, and have used, both....Purely my opinions on them but:
Pros and Cons
ALICE:
PROS:
-Roomy
-Ruggedly built
-Very comfortable and doesn't move around much when hiking or running (with right pads and shoulder straps)
-Long securing straps straps make it easy to secure your sleeping bag to the top.
-Outer pockets are very useful for organizing smaller items, as well as flap
-Fold the waist belt around the back of the pack and secure it, then use it as an attachment point to put an extra canteen/pouch on each side
-Quick release buckles on the shoulder straps work well
CONS:
-Weight. Including the frame they are on the heavy side even when empty.
-Flap is not easily accessible when straps are tightened down
-Water proofing on the flap peels off easily (and probably at this point in time there are not too many used USGI packs that have any of it remaining - so keep whatever is in the pouch in gallon baggies)
-Not as easy to attach secondary pouches/sustainment pouches as other systems (unless you modified it)
-Recommend using a waterproof bag inside of it (really have to say that about all of them though)
-the USGI kidney pad is okay, but the shoulder straps need improvement (easily corrected with Blackhawk or Tactical tailor pads and shoulder straps, but adds cost)
-No specific pouch for a hydration bladder (unless you added an aftermarket suspension or pad system - in my opinion worth it but adds cost and weight) or can always bungee a camelbak to it
-Metal frame that can eventually have some of the rivets pop out (its easy to fix with a pop rivet gun or a small self tapping metal screw...in the field just use some 100mph tape to hold it in place until you can correct it)
CFP-90
PROS:
-Roomy (to a fault)
-Probably the lightest of the choices when empty
-Extra rows of attachment points on the back (not MOLLE, but will work)
-Shoulder pads are well padded and comfortable
-No extra weight from an external frame
-Doesn't move around a lot when moving (then again once its packed full neither do you)
-Patrol pack (if yours has one - in all fairness you can add this to any of them)
-Very large outside pockets on the sides (could use one as a hydration bladder pouch if i remember correctly)
-Straps on top are just long enough to secure a sleeping back (if i remember correctly)
-Zipper at the bottom for accessing the inside of the pack without undoing the top straps (if I remember correctly)
CONS:
-Roomy - I once squeezed into one (along with the patrol pack) the entire contents of two duffel bags...i did have help to compress and hold it in place...weight was ridiculous once it was completely full and only useful for carrying it from the barracks to the truck to the aircraft and back again.
-I feel it is the least rugged of the choices
-Internal "frame" is just really two pieces of long, flat thin metal and easily bent (I took them out of mine)
-Kidney pad/small of your back pad/whatever the heck the little pad in the back is supposed to be sucks and provides little cushion for the small of the back/kidneys
-The channel that holds the little pad is made of plastic and the pad can be pulled out/jump track and (again, if I remember correctly) Its a bit of a pain to tighten the screws that hold the pad in place or they don't tighten well...can't remember exactly, i just remember thinking it was a bad design)
-Outside pockets are large, but few...I didn't feel it was as easy to organize gear in them as on the ALICE
-Strapping on the back was useful for attaching additional items, but doing this keeps moving the center of balance farther and farther back, placing additional strain on your back
-Belt padding was too thin (in my opinion) and its not really feasible to fold the belt around the back of the pack and buckle it to itself so its out of the way.
-Zipper at the bottom (if its packed full then leave it zipped or you are going to have a bad day getting it zipped back up)
-Best i can remember it was advertised as you should put your sleeping bag in the bottom - I think this is a bad idea because it throws off your center of gravity
-Main cavity is huge - don't even look for a waterproof bag for it, just get a couple of thick 55 gallon drum liners (not sure if that is really a con or a pro...seems useful until you are trying to find something that is lose somewhere in the middle of the pack)
MOLLE
(not sure which generation you have - I think mine is the MOLLE II, and its in woodland cammo)
PROS:
-Fairly light weight when empty, lighter than the ALICE (I think anyway)
-Rugged...maybe more so than the ALICE due to the material used.
-Comfortable once set up (After you have found a VCR and watched the video that came with it, and then spend several minuets cursing the strapping system and finally figured out how to set up the shoulder straps and waist belt/kidney pad)
-Out of the three i think the MOLLE shoulder straps are the most comfortable
-Largest kidney pad - seems thinner than an ALICE but I felt it gave better support and was at least as comfortable
-MOLLE webbing...everywhere...you can add extra pouches everywhere
-Very sturdy buckle system on the shoulder straps that with one pull uncouples and makes dropping the pack very easy
CONS:
-You need a VCR (sarcasm aside, if you have no experience setting one up then watch some videos online to save yourself time)
-Frame - cheap, plastic and it will eventually get cracks and breaks in it...seems like the spiderweb of straps and pouches still held it together for the most part, but eventually the frame will need to be replaced
-MOLLE webbing...everywhere...you can add extra Weight everywhere
-I know others will probably disagree but I felt as if the pack moved too much when its on your back...maybe not the main pack itself, but it just felt like there were about a dozen small pouches strapped to my back that would all move a tiny bit as I moved or ran
-Very sturdy buckle system that once broken (or lost) is a bit of a pain to improvise around (so I recommend carrying a spare or two)
-I really felt it could get too compartmentalized once you added all of the pouches and straps...this goes back to needing a large waterproof bag for the ALICE - You need several, various sized bags, for the MOLLE
-Sleeping bag pouch - personal preference, and I was always taught, put the sleeping bag ontop so the weight is pushing down on your shoulders and not pulling backward on your back...but to each their own on that one. i think the sleeping bag pouch is useless in its original place.
There is probably more, this is just what comes to mind.
Personally I prefer the ALICE...that may make a few cringe, but its what I grew up with and what I am most comfortable using and wearing.
I still have a couple of Large ALICE packs that I have set up...the main packs themselves are original, but I either put HSGI AliPads (now discontinued) with TT or Blackhawk straps and kidney pads on them, or one of the older London Bridge Trading ruck suspension systems (which alone will cost you more than the pack and frame together).
I did put together a couple of the Hellcat mods using ALICE frames, medium ALICE packs, MOLLEE sustainment pouches on each side and MOLLE shoulder straps and kidney pads, along with a metal canteen and a 2 quart USGI canteen strapped to it...actually my current vehicle BOB is one of these and I think its very comfortable as well as stays in place well while on the move
However, if i had to pick only one pack it would be a large ALICE with improved shoulder straps, kidney pad and back pad with places for hydration bladders.
Actually you are pretty lucky...if you have all three then fill them up and try them out, as well as try putting together a Hellcat system.