I should probably learn how to do a poll.
Popular Mechanics. They are both fun, but PM did this and The Boy Mechanic (slow PDF) series, which taught us various ways to make ashtrays for Mom and Dad.
Pop Sci. I get both, and read both, but Pop Sci holds my interest for longer, mostly because I’m not a car guy.
I leaned toward buying a PM more often because I subscribed to Scientific American.
Now, I read most of this type of stuff online.
Popular Mechanics was cool, but I always prefered Popular Science.
I pretty much stopped reading both when the relative who had a subscription then passed them on to me died. Wow, that was over a decade ago. 
I did pick up an issue of PopSci a few years back and did not care for it. It seemed like half the issue was highly politicized global warming alarmism. Even without that, it still didn’t seem like quite the same magazine I used to like. Hell, they barely had any ads in the back for cable/satellite descramblers! 
They’re both car magazines. Yeah, I know Popular Science purports to be more general, but still, half the magazine seems to be about cars, and they don’t do all that great a job on their non-car topics.