I have roughly 20k hours in Oldschool RuneScape, and around 5000 in modern RuneScape. This would be a very long discussion if I went into every aspect, but I'll keep it short.

Oldschool RuneScape has a very community focused development process, where what the player says, goes, for the most part. Couple that with the sense of progression and the depth to the mechanics of combat, although at first glance it seems simple, it is one of the most rewarding games you can play. I highly recommend giving it a shot.

While the modern version can still be fun, it is hindered by not being as community driven, having 20+ years of jank built on top of jank, and heavy monetisation (although this is being addressed), just makes the game fall very short of its counterpart for me. While the combat has quite a lot of depth, it doesn't spark that sense of progression or achievement.

Runescape made a lot of changes that people didn't really like, but they mostly just kept pushing forward with everything, and it eventually became modern Runescape 3, which is the non-old-school Runescape. Then, because a lot of people were lamenting how different the game had become compared to the classic experience of the mid-2000's, the developers opened up old school runescape, based on an old copy of the game from 2007 they happened to find.

Over the years new content was added to the old-school version that separates it from Runescape 3, making it a completely different experience, and that new content can only be added if enough of the playerbase agrees to it, making the changes generally positive in the eyes of the players. Old school really feels like a game from the mid 2000's, where pay-to-win mechanics and the like are few and far between, while modern Runescape 3 feels like any other modern game where you either pay your way through or slog through content that seems tailor-made to make you just want to open your wallet to avoid it.

I don't know whether it's better, but every few years when I get a craving to play RuneScape, it's coming from a place of nostalgia; Old School Runescape scratches that itch whereas modern RuneScape would not.

Yes, for reasons that are complicated.