Two fabrics cover most of the bag market. They are not interchangeable, and the choice is usually made for the wrong reason — price alone.
Denier is not strength
Denier measures yarn weight, not tensile strength. A 600D polyester and a 600D nylon have similar yarn mass and very different behaviour, because the fibre is different. Comparing denier across two fibre types tells you almost nothing.
Polyester
Better UV stability, better colourfastness, lower moisture absorption, and it holds a printed or solution-dyed colour well. It is also cheaper at the same construction. For promotional programmes, retail bags that sit under store lighting, and anything with a bright brand colour, 600D polyester is usually the right default.
Nylon
Higher tensile and tear strength per unit weight, and better abrasion resistance. It absorbs more moisture and is less UV-stable, so it needs a coating and is a poorer choice for sustained outdoor exposure in strong sun. For technical packs, straps under real load, and anywhere weight matters, 420D nylon outperforms a heavier polyester.
The part that actually decides performance
The coating. A 600D polyester with a good PU back-coating outperforms an uncoated 420D nylon in water resistance by a wide margin. Specify the coating and its weight alongside the fabric, or you have specified half the material.
A practical rule
Bright colour, indoor or urban use, cost-driven programme: coated 600D polyester. Load-bearing, abrasion-heavy, weight-sensitive: coated 420D nylon or higher-tenacity equivalents. If the bag is outdoors in strong sun for years, polyester regardless of the strength argument.