Fabric - Denier only measures the weight of the thread. The higher the count, the heavier the thread. Solid materials like all nylon or polyesters will hold up better in the seems than the majority of blends. Ballistic just means it uses a basket type of weave in the fabric and it usually resist tearing better. Pull Handles - Look for recessed handles that are not exposed to damage.
Thicker tubing is desired.
The larger the easier they are to zip and have a longer life. Size - Buy the sizes that will best suit your needs for future trips as opposed to buying for one particular trip.