I think you would want an 80/12 topstitch needle for 50wt 2-ply Aurifil. That is what I plan to use. (Superior recommends a topstitch needle for both quilting and piecing, I think.)
I just bought lots of this because I got tired of lint from Mettler 50/3. Have read a couple of threads (no pun intended! :) ) about the Aurifil 50/2. A lot of people use it for both piecing and quilting. My research indicates it is very similar to Superior Thread's Masterpiece, which is recommended for piecing. Superior's King Tut is preferred for quilting by a lot of people, and that is a 40wt 3-ply thread so would be heavier and stronger. Superior recommends a 90/14 topstitch needle for King Tut. Many people use King Tut on top and Masterpiece in the bobbin.
I have not tried King Tut. Thought I'd start with Aurifil because it is about half the price of King Tut when purchased on sale. All of my machine quilting to date has been done with Mettler 50/3, and what I noticed was that close quilting stiffens up the quilt. I am hoping with the Aurifil that the pattern of my quilting will be more at the forefront and that my actual stitches will be less noticeable.
One thing I am looking forward to with the Aurifil is not having to change the bobbin so often! Since it is finer, the bobbin will hold a lot more yardage.