My experiences with these channels are probably completely different than most of you, because I grew up watching them during the late 90s and 2000s, and really haven't watched either channel much (except for Regular Show) in the last nine years. I'll just post some of my recollections of both channels, if anyone's interested:
NICKELODEON
-Rugrats was THE show on Nick before Spongebob became a phenomenon and so had a ton of merchandise. I had toys, computer games, school supplies, books, and clothes based on Rugrats and still have some stuff in storage somewhere. Nowadays, I'd be surprised if there's any new Rugrats merch to be found, except online (still really limited, though.) Of course, Spongebob became much bigger than Rugrats ever was.
-I remember watching both Fairly Oddparents AND Invader Zim the day they both premiered in 2001, so I'm an original Zim fan and also followed Oddparents until around the time the live-action movie came out. I've heard that they added a dog fairy and a new girl to the main cast, which seems like a baffling move when the show is so old.
-You know what? KaBlam was a kickass show. Nickelodeon REALLY needs to sort out whatever licensing issues there might be and release the show on DVD.
CARTOON NETWORK
-I really loved CN back in the day, but it always felt, I don't know, cheaper? than Nick. I think it's because most of their shows seemed to have lower budgets than contemporary Nicktoons or had that budget Hanna-Barbara style (actually, I think HB produced a lot of the shows). CN also relied more on reruns of Tom & Jerry, Loony Tunes, and Hanna-Barbara cartoons, so they probably had less original shows than Nick. CN really began to pick up in the early 2000s, so that look of cheapness applies more to them during the 90s.
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Cartoon Cartoon Fridays was a huge deal for me. It was basically a huge night of cartoons when they'd air new episodes and characters from a different show would host it each time. CN's cartoons also had this epic opening before most episodes and I was disappointed that Foster's Home (which aired when the Cartoon Cartoon concept was scrapped) never had a Cartoon Cartoon sequence.
-I would guess that CN's more obscure shows (Whatver Happened to Robot Jones, Sheep in the Big City, Mike, Lu and Og, etc,) are probably more obscure than older Nicktoons, for whatever reason.
-Something that I forgot until recently and that seems weird in retrospect is that Grim Adventures of Billy & Mandy was originally half of a show called Grim & Evil, which split into two shows: Grim Adventures, and Evil Con Carne (which lasted a single season).
-i also remember when Adult Swim was first launched and I did watch some of the shows even though I was way too young for most of them (though Space Ghost Coast to Coast was inexplicably put on Adult Swim, even though it had been on Cartoon Netwrok previously.)
One weird thing about Adult Swim, and this may just be my faulty kid perspective at the time, is that it seemed to be somehow tied in with a Codename Kids Next Door episode about a literal adult swim. Granted, it would be really odd to tie advertising for an adult programming block to a kid's show, and there probably was never any such connection, but that episode of Codename: KND aired at the same time Adult Swim was launched and I definitely seem to remember some sort of reference between them. I just looked up the episode,
the pilot, and other people noticed this too, so maybe the advertising was intentional.