Here are five 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 was first manufactured).
Ease of Packing
When considered ease of packing, look at the main compartment zipper orientation, the number of pockets, size and the shape of opening, and how hard it is to get large items in and out. Also, consider how hard it is to search inside for items. For zipper orientation, in general the D-shaped openings are better than strait zippers because they allow you to see and access more space. The North Face Base Camp Duffel was one of the first to have the D- shape opening but now many duffels do. If you have long items such as two-section trekking poles, there may only be a few models that are long enough. The number of pockets and how easy it is to access those pockets is also crucial. In general, an easily-accessible pocket in the lid is ideal.
Ability to Carry and Transport
Consider how pleasant it is to carry the bag 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. All backpack straps have varying levels of comfort depending on how thick the foam is and where they attach. A bag like the Gregory Alpaca Duffel has all the options: backpack, carry straps and an over-the-should strap. Also, consider how easy it is to remove the backpack straps. Some bags have nifty ways to tuck away the straps without needing to remove 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 generally 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.
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 most burly 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.