The "d" stands for denier which is a term that describes the "thickness" of the fabric.  The 20d is a thinner fabric, and hence lighter weight and less durable than the 33d.  The waterproofness rating is the same for the 20d and 33d.   eVent is a breathable fabric so in addition to the venting the Packa provides, moisture on the inside can escape directly through the fabric.

Weights: 

small 20d:  9:00 oz           medium 20d:  9.5 oz            mediumx 20d:  10.oz         large 20d:  10 oz          largex 20d:  10.5         Price: $ $142.50 + shipping
small 33d:  12.50 oz        medium 33d:  13.0  oz         mediumx 33d:  13.5.oz     large 33d:  13.5 oz      largex 33d:  13.75     Price:  $122.50  +  shipping

Roughly, you save 3 oz with the 20d, and pay $20 more.

small eVent:  18oz             medium eVent: 20 oz         large eVent:  22 oz