There is no best haul bag for every application – no one size fits all. So before you buy a bag think hard about how often you are REALLY going to use it and for what. Just going to do a wall or two? Mainly into fast and light ascents? Want a bag that also doubles as a crag pack? Once you know exactly what you are using your bag for it is more clear what to buy.
There are roughly four sizes:
Haul packs: 1,500-3,000 cu inches
Small haul bags/haul packs: 3,000-5000 cu inches
Medium haul bags: 5,000-8,000 cu inches
Large haul bags: 8,000-10,000 cu inches
The size of the bag you want depends on both the type of route and how long you will be up there.
For The Nose I like to have one large haul bag so that everything fits in. On The Nose, where there are a lot of low angle pitches and lower-outs, you don't want multiple bags or a lot of stuff dangling from the bottom of the bags. The more stuff that dangles the more stuff that can get caught on roofs, corners, or the haul line itself. Keep in mind there is a big down side to big bags: when they are taller that 32 inches you really have to dive to get stuff in the bottom. It becomes hard, especially at the top of the route, to find that last bottle of water. Also, look out for any bag that feels too narrow. In general, haul bag manufacturers make the bags too long and narrow, which makes it hard to access the bottom, especially at a hanging belay. Look for bags that feel wide relative to their height.
In contrast, on a route like Zodiac where many of the belays are hanging, it is nice to have two medium-sized bags because it is easier to access what you need. You don't need to worry about the bags getting hung up because the wall is so overhanging.
Small haul bags are sometimes the most versatile size and sometimes totally unnecessary. They are nice for overhanging routes if you can fit everything into one of these and one medium bag. They excel at fast and light walls like the Regular Route of Half Dome (that is if you can get everything to fit inside). I used a small bag on a one-night ascent of the Salathé Wall on El Capitan but I packed exceptionally light, brought tiny sleeping bags, and climbed it in August. A small haul bag is the least essential bag for big walls but it is the most useful for non-big wall activities. They have great crag packs because they fit in a ton of stuff and are generally free-standing. I use a small haul bag to deliver all SuperTopo books to the post office.
There are two main types of material: vinyl coated nylon and urethane (Metolius calls their urethane Durethane). In general, we prefer urethan/Durethane. It is the stuff river rafts are made of and is the most burly. Yes, you can put holes in it if you don't pack your bag right and haul low-angle slabs…but you have to work at it. Urethane also doesn't seem to break down as much over time whereas vinyl seems to get more crackly and susceptible to wear. Vinyl is generally less expensive and also lighter. If you are doing a very steep route, vinyl is no big deal. If you are doing a low angle route, you need to take more care to pad the inside your bag with a foam sleeping pad and made sure there are no sharp objects. We prefer urethane but have used plenty of vinyl bags and never busted one open mid-wall. If you climb less than five big walls, the durability doesn't even come into play.
When it comes to closure systems there is a trade off: the more watertight the closure system, the more material you have to manage and therefore the less convenient it is to access the bag. The most waterproof seal is a "river bag" style closure, which means a big skirt you have to roll and unroll every time you want to access the bag. Since I didn't experience my first big wall storm until big wall ascent number 103, I feel the trade off for having a less waterproof and more convenient opening system can at times be worth it. Also, the truth is that no closure system is truly waterproof if you are in a serious wet storm with runoff. Plus, you need to have the bags with river bag closures pretty full and closed just right to keep a puddle from forming that eventually soaks through. Even then, water can still get through. So if you are serious about keeping everything dry, everything needs to go in a true dry bag inside your haul bag. At the very least, you want a river bag for your sleeping bag and synthetic layers.
One nice benefit of a closure with big skirt is that it makes a great improvised bivy bag on the summit. I have probably spent 30 nights on the top of El Cap with my legs in a haul bag and every extra inch keeps you that much cozier.
Also, the closure system is either made if denier nylon or ballistic. Ballistic is the more bomber and waterproof.
Seams and stiching
All haul bags have a big seam down the middle. There are two ways to deal with this: weld the seam or sew the seam. Welded seams are the most bomber. Of all the haulbag failures I have had, it was always a sew seam that blew out. That said, this was often after the bag had been up 15-plus walls and wasn't always cared for properly.
All haul bags now have pretty similar suspension systems that tuck away when not is use. I have long debated which is the most comfortable, but in truth they are all equally uncomfortable when you have 80-plus pounds on your back.
All haul bags should have generous inside pockets so you can easily access sunscreen, a snack or two, and your headlamp. All haul bags I have seen on the market have these pockets. Some also come with internal daisy chains. I have always appreciated this feature, but by no means think it is mandatory.
Here is a great Do it yourself big wall thread and some SuperTopo Forum Pig recommendations