CN spent most of the 90s as being predominantly reruns of boomer era shit, REALLY bad Hanna-Barbera cartoons, and maybe an hour or so block of original programming such as Dexter's Lab or EEnE. It wasn't until the early 2000s that CN began to catch up, but I still give Nickelodeon the slight edge.
The period between when Airbender ended and Loud House began was the only time CN really truly was ahead most of the time, but I feel as if both were overshadowed by Disney Channel at that time.
Funnily enough, Cartoon Network used to be the edgier, more "late grade school early high school" channel, but has pretty much become FisherPrice Network. Nickelodeon for the most part has stayed the course and remained decent, though I feel they'd be back to 90s level strength if they had actually picked up Glitch Techs instead of relying so much on TLH and Spongebob spinoffs.