This question is a little awkward to answer because in it two unlike things are equated for purposes of comparison. In actuality, "ripstop nylon" is the fabric and "1620 Denier" is the measurement of the strength of the ripstop fabric. These are not two independent fabrics: Only one is the fabric and the other is the measurement unit of the fabric strength quality.
Ripstop nylon has a crosshatch weave wherein nylon taffeta is woven with double yarns that create a box pattern of crosshatches that are lightweight, durable and that resist snagging, running and tearing. (Whole Foods Markets sell great shopping bags in several colors that are made of this great ripstop nylon product.)
The measurement unit used to ascertain the strength of the ripstop is called, as we have established, the Denier. It is graduated upward so that, for example, 600 Denier is less strong than 1200 Denier, which in turn is less strong than 1620 Denier. The softness versus coarseness qualities correspond directly to the Denier increments. This means that 600 Denier is more soft and less coarse than 1200 Denier and, likewise, 1200 Denier is softer and less coarse than 1620 Denier. These numbers are examples of strength and I am not implying that these three numbers are the only increments for ripstop nylon Denier measurements. I only indicate that graduation upward in measurement value corresponds to upward strength and increased coarseness.