That may be a false dichotomy.
Remember that Call of Cthulhu is a *horror* game. Can we agree that achieving some sensation of horror is one (possibly among many) implicit goal of play?
Things that achieve the goals of play are the substance, are they not?
It's a dichotomy in that I think most games have both, sure. But I think that there are examples in other media we could point to where how the story is told is more important than the story itself. Something like Mulholland Drive, let's say....or most of Lynch's work, to be honest.
Do you think this exists in RPGs?
Well, of course you can play Call of Cthulhu without these things. But I counter the question with a question - How likely are you to achieve horror without these things?
Same things goes with humor, for games like Paranoia, or Toon. Being *funny* is a significant part of the point of playing. Do you want to try to be funny without putting consideration to how you present the material?
I think that horror is tough, honestly.....I don't know if trying to establish a spooky mood through language often works in RPGs. I think it can, for sure. I think it tends to help when there's already some kind of content that's unsettling in some way.
As for humor, while I've never played Toon or Paranoia, there are always lots of laughs in my group's RPG sessions, and most of them are due to spontaneous comments more than any kind of craft.
But I think this also kind of raises a good point.....what about a player in such a game who just isn't that funny? Maybe they love humor, and enjoy comedy, but just aren't that great at being funny themselves. Or aren't that comfortable with it? What about a player who doesn't have a strong sense of theme or mood in fiction? Do these players not enjoy the game as much as other players? Are they not playing as well?
Are these skills essential in some way to RPGing?
I don't see a particular need to make the generalization, to be honest. We could speak about particular moments during play, when we could say that one perhaps should take precedence over the other. But, I don't at this point see much value to be found in proclaiming that, over play in general, one is (or should be) predominant. But, I'm game to hear some ideas.
What do we gain, in our consideration of our hobby, from proclaiming one over the other?
I think that some of the comments in this thread have been enlightening, when it's not been sidetracked by argument. Some of the questions you and I are discussing now are very interesting to me. I feel like I get a better understanding of games and gamers. As someone who has primarily played with some iteration of the same dozen or so people over the years, I like to get views beyond my own. I think my group is pretty typical in many ways, but sometimes I learn that we may not be all that typical.
Ultimately, what's to be gained with any analysis? Those who accept the OP's premise may be able to focus their efforts on what they think is essential to RPGing. Those who don't accept the premise, perhaps can see an area they usually don't focus upon.