I have now spent a lot of time testing out various models of Mystery Ranch backpacks (mostly the tri-zip models) and so I thought I would do a quick comparison of how you might think about which is right for you. Here you go, hope it helps someone:

UA21, ASAP, 2 Day Assault, 3 Day Assault CL

UA18 and UA21

• Urban Assault 18 ($125 retail, as low as $68 on sale) Review: If you want the best everyday backpack for stuff, and your are shorter, this is what you want. This is like the Urban Assault 21 but better fit for smaller people. The EDC choice for the under 5’8” crowd, with an outside chance it will work for your kid (target about 48” minimum height to make it work). My kids love theirs, and it’s a really solid bag that leaves little to complain about.
• Urban Assault 21 ($125 retail, as low as $68 on sale) Review: If you want the best everyday backpack and you are not shorter, this is what you want. This fits almost anyone well enough, offers insane value, and is overall a great choice for an all around backpack. If someone doesn’t want to think about backpacks but has a $100 budget for a bag, this is the backpack I recommend. It’s hands down underrated and silly good. Hiking, office, travel — it will handle pretty much everything really well.
• 1 Day Assault Fixed ($199 retail, can be more on eBay) Review: If you want a little more tactical/outdoors vibe to your bag, or simply want USA made then this is a silly extra waste of money over the Urban Assault 21. Amazing looks and features, but doesn’t fit as well as the 21 for some reason. The price is stupid, but it is made in USA and looks can’t be beat. If you want a UA21 that feels slightly better and is made in the USA, then this the one to go for. It’s far from perfect, but still quite good. Generally I recommend avoiding this as the UA21 does it a little better while being half the price.
• ASAP ($280+, I’ve never seen this on sale) Review: If you have no budget, don’t care too much about rolling your own laptop sleeve, and simply want the most comfortable small backpack on the market — I’ve yet to see anything that can beat this. If you want MOLLE, this bag is for you. The adjustable yoke makes it a dream to wear, but the looks suffer a bit on this bag compared to others. If you like the looks of this bag, there are few backpacks out there which will carry gear as comfortably as this one. The straps are durable and comfortable and the bag is USA made. It’s not cheap, but the price shows in the item you get. Toss a device sleeve in it and you are good to go, this is my go to bag these days and it is fantastic. Of the sub-25L bags on this list, this is handily the best quality most comfortable one. Not the best value though.
  • Front ($120-160 used, discontinued) Review: Despite being discontinued, this is the bargain hunter’s pick on the list — find a good deal on a used one and you have a great bag that is slightly not as good as the ASAP, all while looking better. The thing is, the Front is better than the 1 Day Fixed, and better than the ASAP. Not better in every way, but the value proposition is huge, and you get the looks and features which offer the best mix. You simply can’t find many of these in good shape, but if you happen upon a good deal, then these bags are great. If you have an ASAP, it does have much better straps, so the Front might feel like a downgrade to you.
• 2 Day Assault ($199 retail, often discounted to $150 places) Review: If you want the ASAP, but bigger, then yep, this is the ASAP made bigger. It has the best laptop compartment of any Mystery Ranch bag I have tested/seen. As it offers both the normal top load inside the bag, but also an external access zipper. This bag often is listed as 27L because that was the stated volume when it came out, but Mystery Ranch has since revised it to 29L — which is the reality. It’s a big bag, but it is really good. If you want a single bag to do almost everything at a good price, this is your pick. But know what you are getting: big bag, solid value, comfortable.
• 3 Day Assault CL ($399 retail, I’ve not seen it on sale) Review: If you have no budget, and you only want one backpack period, this is that bag. This is what I would recommend, and if I were told to slim down to one bag, this is that bag. It’s the 2 Day Assault, slightly larger, USA made, and amazing. The USA made Mystery Ranch bags feel much better, with softer Nylon, and beefier hardware on them. This 3 Day also has a storable hip belt which is fantastic and the only regression from the 2 Day is the lack of external laptop access. But that’s offset by less MOLLE on the bag — a welcomed trade off. This is truly a show stopper of a bag, and the pricing reflects that.
• Scree 32L ($189 retail, I’ve seen it drop below $150 on sale) Review coming soonish: this is the hiking version of the 3 Day Assault. Very similar sizing and setup with a few tweaks. There’s no laptop compartment, just a place for hydration. The top lid is the same, but instead of zippered pass through for the bladder hose, it is a slit to maneuver the hose through. But the biggest change is that it is 210D Robic Nylon, not 500D Cordura Nylon. The difference is massive. What’s 210D Robic: it’s the ’normal’ thin and silky feeling nylon backpacking companies have been using for ages. Is it as durable as 500D? No. Is it nearly as durable as 500D for the average person? Probably. It’s a good deal though, and the hip belt is much more robust while being completely removable instead of just storable. Assuming you get one for the $140ish price, it is an outright bargain.

For me, I’ve keep Urban Assault 18s for my kids, a 21 for my wife, and an ASAP and 3 Day Assault CL for me. A near perfect setup for me. (The Scree 32 is my Bug Out Bag.)

Scree 32 left; 3 Day Assault CL right.