Chris, as you know I've got experience with both fabrics, I'll give you the straight answer regarding durability: In this application, they're functionally the same. If the wt savings justifies the cost difference to you, it's a no-brainer, if it doesn't, ditto.
The tight weave of the 20d makes it more snag resistant in my opinion, but the 1.1 being type-66 nylon signifies a measurably higher strength for applications such as hammock bodies.
The BWDD has the best feel of any 1.1 I've touched, but the 20D Momentum/Intrepid feels as good as a textile can IMHO. The difference between the RS and the Taffeta is just Cashmere vs Velvet IMHO, a matter of opinion RE: luxury.
My main hammock quilt is in full 20d RS, which I've recently decided I favor over my 20d Taff ground quilt, but I think it's just because the RS is less "feely".
BTW, that sounds like alot of fill for such short baffles. I recommend cutting the baffles to desired loft height, and then sewing them 1/4" from edge to your shell fabric, that should give you baffles ~25% shorter than the target loft.