Kamen rider is not Power rangers for adults. Power Rangers is originally a japanese series called SupeR Sentai. All the robot fight scenes come from it.
Kamen Rider still panders to younger audiences, selling merch (each new kamen riders a gimmicked belt that he or she uses.) The show is still colorfull and painfully japanese with all the lovely poor CGI you can ever want. But I still say give it a shot. ITs pretty gloriusly hammy in the best of ways.
I know both are merch cash cows with a "general Japanese audience" mindset, but actually, I get why his friends would say that. I've watched Japanese seasons of more than a couple Super Sentai series and Kamen Rider series and even more of the American stuff.
One of the things I noticed immediately is that in Super Sentai, the cheese factor seems a lot higher on the one hand. But on the other, it's also more melodramatic. There are more instances of villains portrayed with some sympathetic qualities but NOT surviving and played for tragedy(whereas in Power Rangers, most of these kinds of villains will get to survive). Also, in Super Sentai, they explicitly fail to save every civilian. People running from the monsters sometimes die in front of them onscreen, and the rangers get upset about it, though they generally avoid playing it too heavy. And all though later series of Power Rangers finally did start incorporating this element, the idea that even a Ranger can really be killed off before a series ends has been present for longer in Super Sentai. In spite of all of this, it can still come across as more upbeat than the American version because, again, the colorfulness and cheese seem so much more earnest.
Japanese Kamen Rider, depending on which iteration, does feel darker and more "adult" to me. I've read that shifting writers between them is a big reason for this. There seems to be a lot more moral ambiguity. Fewer central authority figures. More genuine angst over searching for the right thing to do and dealing with fallout from monster's rampages. Heck, even the Riders will end up as antagonists to one another for pretty long stretches WITHOUT it being a simple case of "well, one of them is evil/brainwashed and once the good one has that one long, good fight with him, they'll be teammates for good until the next time they get mind controlled." Kamen Rider Ryuki had Kamen Riders trying to kill each other as a key element of the plot!
I also note that sometimes even the relative "grittiness" of some Kamen Rider series to Super Sentai can even be seen in the fight choreography. Super Sentai choreography always seems to have a bit of a stylish, classic kung fu movie polish to all the moves. Some Kamen Rider series fights are like that, but sometimes I was stricken by how some of their fight scenes involved a less structured feel. Like you'd have some karate-like movements but also some inelegant looking grappling that sort of added to the sense of struggle.
It all makes sense though: Even though Kamen Rider is still a kid-friendly show, it
is closer to Ishinomori's original, darker vision(see the anime, Skull-man for some idea of what it was originally like) when he unintentionally more or less created the genre by cleaning up the ideas. Super Sentai was inspired by it rather than being a more idealistic adaption, so it being the less "adult" of the two makes sense to me.
That said, it too has some cheese to it. But watching them back to back definitely felt different to me.
