Popular Science and Popular Mechanics are pretty much interchangeable. You will see some articles about new tech, or future tech that may or may not ever happen. You'll also see the gratuitous stuff they put out there to sell magazines. It's not anything you'll sink your teeth in and once you've read it, you can be pretty safe in recycling it without any real loss.
This is a change from what it was 10 years ago with projects and references you'd want to keep around. They both were great project and resource magazines. Today they are both more airline trip fodder. Just enough to keep you mildly interested, but limited to about three pages for any given story.
I chalk it up to a different audience focus.