Ok, my package of Red River Polar Metallic arrived today so I could finally do my own side-by-side comparison. I'm working off 8.5x11" cut sheets, not rolls.
At a high level the two papers are virtually indistinguishable. If you didn't see the two papers side-by-side on a wall nobody would be able to tell the difference between the two of them.
There are, however, some extremely subtle differences, at least in the samples I tried. My tests were using a set of 4 black and white images printed 4-up, as well as the standard test print image I use for my inkjet paper list
Here's the differences I see:
1) The base paper has a different tone. The Lexjet paper is ever so slightly more blue.
2) The Red River Polar Metallic has a very, very, very, slight increase in texture. It's still a smooth paper, but there's an orientation to the fibers of the paper (landscape on the 8.5x11" cut sheets I have), and it is very slightly more pronounced on the Red River
But these are extremely
minor differences. In terms of the metallic quality and "wet look" both appear equal, and both are nice and a fun change from standard offerings. I asked my wife to confirm in a random arrangement of the sample prints, and she was also unable to see any difference between the two with respect to metallic and wet appearance.
When compared to Kodak Endura Metallic (which I also placed side-by-side), the Kodak is still king when it comes to metallic and wet look. It's also much more heavily textured on the base layer than either of the two inkjet offerings.