I found a paper from 2013 that compares storage densities up to 2012:

http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6...

In 2012 they list the density of magnetic disks as 750 Gb/in^2 and nand flash as 550 Gb/in^2. I'm not sure how the numbers have changed with 2d nand, but 3d nand probably pushes the density way over magnetic.