Here are a handful of key things to consider when buying a burly travel Duffle Bag (or more accurately called, "Duffel Bag" for the town of Duffel in Belgium where the classic bags were first manufactured).
Volume is easily the most subjective need among travelers. Climbers, skiers, divers or other people traveling with an abundance of extra equipment will often need more space than those going to visit family for a holiday weekend or a warmer place with fewer layers.
For general guidelines to help you make your selection easier: most people find a model in the 70-120 liter range works for most of their extended trips (more than 3-4 days). Even for the most experienced of travelers, carefully packing takes diligence when trying not to exceed the 50-pound limit.
A 90-100 liter bag is much easier for the majority of travelers to fill and not have to exercise as much care to stay under 50-pounds. On the smaller end of the spectrum, a bag in the 70-liter range will fill up quickly; however, it does work well for trips where you don't anticipate bringing a lot of extra equipment or for those going on adventures to warmer climates.
Ease of Packing/Unpacking
When considered the ease of packing, look at the main compartment zipper shape and orientation, as these will both have a significant impact on getting items in and out of the bag. It's also a good indicator of how easy you'll be able to close it. In general, the D-shaped zippered openings are better than vertical zippers because they allow you to see and access more space.
The number and design of pockets help with organization and can save frustration by helping their user keep track of easy to lose items. More pockets aren't always better; some pockets are particularly difficult to access, especially when a bag is full. In the individual reviews, we break down each model's pocket designs and how useful they proved.
Zipping Closed When Full
While zipper placement and design play a significant role in a given model's ease of packing, it will also impact how challenging a given model might be when trying to close, and when packed to the max. Zippers that close with a flap on the top or those with generally stiffer sides were probably to close. For example, the Eagle Creek Gear Warrior Wheeled and the Osprey Ozone Convertible were far more challenging to zip closed when full than the Patagonia Black Hole Wheeled or The North Face Rolling Thunder 30".
If you have long items such as two-section trekking poles, tripods, or other lengthy activity specific equipment, there may only be a few models that are long enough. It's worth paying attention to a given model's length to save some frustration down the road.
Ability to Carry and Transport
This category breaks down into two major categories: wheeled models and traditional duffel bags. Wheeled models are often easier to move and are better for excursions where the ability to roll luggage is easier than carrying it. The obvious caveat to this is the destinations where you are traveling need to often be less remote; they will likely need to have paved surfaces, for the most part, and the capability to lash bags to vehicles or pack animals isn't a much of a factor.
Besides versatility in moving across the terrain, the other substantial downside to wheeled luggage (compared to traditional duffel bags) is they are heavier, often between 4-7 pounds heavier. Meaning that's 4-7 pounds of stuff that you'll never get to bring.
While generally speaking, traditional duffels are more physically demanding to travel with as you have to carry them versus roll them, they do offer a lot more versatility to trot the globe with. They are probably easier to move across more uneven terrain and any non-paved surface, they are also much more easily attached to sleds, snowmobiles, pack-animals, vehicle roof-tops, helicopters and dozens of other non-traditional modes of transportation to say the least. Additionally, there are several small planes across the globe (3-8 passenger small, not a 737) that don't even allow you to bring hard-sided luggage.
However, not all duffels are created equal in regards to their ease-of-transport. Consider how pleasant it is to carry a given model in three modes: in your hand, slung over your shoulder, and worn on your back in backpack mode. Some bags do not have backpack straps, which is a bummer if you are going to take long hikes through airports. Because none of the burly travel duffels we tested has wheels, these backpack straps can be crucial.
Nearly all the models with shoulder straps were reasonably comfortable to carry. A couple of standouts were The North Face Base Camp, Gregory Alpaca, and the Patagonia Black Hole, which was exceptionally comfortable. Another feature to keep in mind is how easily it is to remove or stowaway the shoulder straps. Some models have nifty ways to tuck away the straps without needing to remove them, quite a useful feature to help to keep them from getting chewed up on conveyor belts when checking them.
Good lashing points and compression straps also are essential for ease of attaching to buses, mules, and just fitting in tight spots. Duffels that can be carried, lashed, and stashed in the most ways are the best. Also look for models with grab loops at each end for pulling them out of buses and trucks or just dragging them around when you have too much stuff. Overall, the easiest way to carry a duffel is in backpack mode so the backpack straps' comfort and functionality is the most important thing to consider.
Wheels Not all wheels are created equal, generally speaking, the larger the wheel, the easier it is to pull over uneven surfaces like gravel parking lots or cobbled streets.
When looking at the durability, we took into account the material each duffel was made of, how burly the seams were stitched, and what kind of zipper they used. Most bags are made of pretty thick polyurethane, which is considered the burliest and water-resistant material. However, other materials like ballistic nylon are also often burly enough (but not as water resistant).
Weight is a big factor now that most airlines limit you to 50 pounds per bag or if flying outside of the States, 22 kilograms (48.5 lbs for the metric uninclined). Every ounce matters, which is another reason many people go with duffel bags—they are seven to nine pounds lighter than comparably-sized wheely bags.
This is another important function of your duffel whether it is riding on top of a bus through a rainstorm on the Karakorum Highway, hanging in a tree keeping critters at bay in Patagonia or anywhere else you might venture. It is also nice for Alaskan expeditions where people often put a duffel in their sled.
This is less of a big deal now with TSA only allowing certain locks; however, when traveling to more remote regions or on trips where you know you'll drop your bag at a hotel for extended periods of time, locking zippers add a little peace of mind.
Two posts (instead of a single post) on a bag's retractable handle make stacking a second (or third) duffel much more stable and easier to manage. This is true regardless of if it's a light carry-on, purse, backpack, or a fully loaded second 50-pound bag.