Re: 5.11 72 hour backpack.
If you take a close look at all packs no matter who makes them whether in US or overseas, all identical and come from the original design of Greg Lowe mid to late 70s design. I used an original Lowe Alpine clam shell for years early 80s. Clam shell packs by their design are not as weatherproof as those with a top lid cover. 5.11 uses a seam bead on each side of the zipper that makes using the zipper hard to pull until it loosens up, not good weatherproofing as nothing covers the zipper. A nylon flap cover over the zipper allows some weatherproof. Waterproof zippers bend as you are wearing the pack and can seep water in also. Clam shell packs design allows some water in, the zipper is on top and takes all precipitation directly. In really wet conditions, I always line my pack with trash compactor type plastic bag. I am not sure who or why clamshell packs are considered tactical packs but these days everyone has a knock off of Greg Lowes original design.
5.11 really well thought out this pack for stashing / organizing kit in a pack. More of a travel bag type to me than what I prefer with all the inside pockets, mesh, zips, etc but if this is what you like then good. I like the carry capacity of the 72, could get out for a week or so. The main compartment clam shell zips open deep so access to kit inside is a plus. Down side is zippers are the weakest link. I have seen zippers fail. The side pockets get covered if lashing kit there so...either use the pocket or do not or remove what you have lashed. I like to lash stuff to the side of my pack. The little stash pocket on top of the main compartment, some like some do not but as long as your team mate can access it, it works ok. I usually carry small pockets on my shoulder straps for fast accessing kit, no need to ask anyone or remove my pack. The glass pocket is nicety but does nothing to protect your glasses unless you use a hard case then no need for the lined pocket.
The beaver tail is loaded with mesh, pockets, inside for just about anything to include some 308 10 rounders. Zips open near flat. Beaver tails are good for carrying heavy / odd sized kit; spotting scope with tripod, e tool or snow shovel, firewood, game, water jugs, showshoes, etc. but do collect trash; rain, snow, twiggs, leaves, sand, needles, etc in the slot between main and beaver tail.
Waist belt folds up out the way. PALS / MOLLE on the waist belt is useful.
Carrys average, but if you want the pack to bend and move as you, I like just a plastic sheet.
Vent holes are to drain water out. Not much gets in even if placing pack in a pool of water / slush, like an upside down glass in water but a bellows effect while opening the pack will suck water in the holes if placed the bottom of the pack in water. You can always plug them up. I think they are on the large side.
I am currently using Camelback clam shell design, MotherLoad. I paid new $146 multicam from a vendor here on the hide two years ago. Very well thought out and made clam shell pack and is very close to Greg Lowes original design but with PALS/MOLLE for the tactical look. The main zip could zip open more, when the pack is loaded it is more difficult than it could be to access. It has a few slots and pockets inside for tools, data book, binos, etc but not too much. Never used this one in the rain but have snow and nothing got wet. Its been drug and dragged a little, since I am old and retired now I do very little exertion work.
The 3 Day Assault Plus from Tactical Tailor looks superb, what I wanted until the CamelBack came along $100 less and I understand TT will modify it some but the cost is over your budget.
For the average user, I would not hesitate on the 5.11 Push72 if you like it, it should last many years.
Good luck