I was around for both of these. The media frenzy in both cases was crazy. I’m going to say in the case of Twin Peaks, it was just a little crazier, mainly because everyone assumed the big reveal would come at the end of the first season, and it didn’t, but also because there were lots of clues dropped in the episodes, so it seemed that people ought to be able to figure it out.
There was tons of discussion because there was tons of theorizing, and “proof-texting” for lack of a better term, to back up one’s theorizing.
But also, “Who shot JR?” happened in 1980. A few people had VCRs, but there weren’t many out there, and since no one knew what was in the offing, people had not taped the whole season in anticipation of a mystery at the end.
On the other hand, Twin Peaks was 10 years later, and directed by someone famous-- really, really famous, just having come off of Blue Velvet. We kind of had an idea that this would be a protracted mystery, and at any rate, it was directed by David Lynch. Moreover, pretty much everyone had a VCR by 1990. So there were lots and lots of tapes of the whole season circulating after the fact.
Basically, way more people watched Twin Peaks than Dallas. Even with summer reruns. And getting together and watching Twin Peaks as a group was a thing. I don’t think Dallas groups was a thing, albeit, I was in jr. high/a high school freshman for Dallas, and just out of college for Twin Peaks. But I also know my parents and brother watched Twin Peaks, and never Dallas.
It’s also true that a book called The Diary of Laura Palmer, authorized by the TV production company, and written by David Lynch’s daughter, became a best-seller over the summer between the first and second season of Twin Peaks, and there was a whole cottage industry of T-shirts and bumper stickers. There were some T-shirts and even a novelty song about “Who shot JR?” but all-in-all, it didn’t generate what Twin Peaks did.
However, without the JR phenomenon, I don’t know if people would have been “ready” for Twin Peaks. End of the season cliff-hangers weren’t really considered “fair” before Dallas, but now they happen with nearly every shot, all the time. If it hadn’t been for Dallas coming before, I think people would have been much, much angrier about the cliff-hanger at the end of the first season of Twin Peaks, and there could have been a sort of backlash that stopped people watching altogether.