The RRDS is a better jean, period. The Greycast is a great jean, but if you have the money, go for the dry selvage. I have a pair of both, and I would only consider getting a second pair of the selvage.
I have to agree with regularRyan. The RRDS just feels like a tougher and more rigid jean. Although I love the color of the Greycast. I bought the RRDG online because of the price, but today I saw the RRDS in person and couldn't resist buying them.