Here is a link to a comparison between the i1Display Pro and the DISCUS at the Dry Creek Photo website:
drycreekphoto.com/Learn/Cali … _i1D3.html
Unfortunately this was done before the Spyder 4 came out, so you don’t have that for comparison.
If you really, really want accuracy, you could go with the spectroradiometer mentioned in this article:
drycreekphoto.com/Learn/Cali … dware.html
(at something like $13,000)
I have not had a chance to compare a Spyder 4 with an i1Display Pro directly - so I can’t give you a definitive reason to prefer one over the other. As I say, I would tip the scale toward the i1Display Pro mainly because of the good reputation X-Rite has with their instruments, and I have seen it produce very dark black measurements (so it should do well with representing shadows).
It really boils down to how important accuracy is to you. For most people, an i1Display Pro or Spyder4 does a good job and keeps their displays calibrated so they can get dependable color they can count on. But there are those who need and want something more in terms of accuracy, and that’s what the DISCUS was made for. It not only is more accurate, but more consistent month after month, from one calibration to the next - and it is made to last, with filters and materials that will not deteriorate over time.