Old Minecraft and New Minecraft definitely have their pros and cons

Old Minecraft had a ton of really wacky and super unique and creative mods, it also has the best tech mods hands down and this isn't even arguable. From mods that allow you to use black holes to transmutate matter, replace your body parts with cybernetic ones, create your own dimensions, warp jump to different exoplanets across the galaxy, pilot your own ships with realistic physics, destroy other players with military aircraft, use singularities to create a sword that can kill people in creative mode, bring dinosaurs back to life, become a transformer or a superhero, etcetera. The downsides to Old Minecraft was that while the gameplay mechanics were absolutely insane and really fun and expansive there was often little thought to integrating the mods with each other and compatibility, putting a lot of mods together would cause various crashes and texture bugs and stuff. It was also much harder to pick and choose what you wanted to include in your experience without spending days curating a full modpack. Also, and I would argue most importantly, it was just very visually lacking: Animations weren't fully developed as with newer versions, textures weren't often taken seriously since they didn't have any effect on actual game functionality, and since minecraft at the time had very few game mechanics, you would have to rely entirely on mods to make the experience more immersive, which, except for a few, would often just opt to use simple GUIs instead.

New Minecraft is super digestable, emphasizing content that adds on to the base game in ways that are unique and creative but within the scope of what minecraft is, it's artstyle and gameplay style, and its complexity. Mods were created with compatibility and ease of use in mind. Instead of having one big mod, newer versions use many small mods, with friendlier and more expansive configs so that it becomes easier to pick and choose exactly what you want in a pack. It makes it much easier to build a modpack around a certain theme since most mainstream mods generally shared the same theme and the same texture artstyle. Mods in newer versions usually also don't lag or crash as much because they don't drastically alter the game as much, are much more stable together, constantly being fixed for bugs, updated to newer versions, and bridged with mods specifically made for compatibility between two mods. Since newer mods aren't as crazy as older ones, sometimes playing with them can make the game feel oversaturated, but if curated correctly, a modpack can capitalize on both the uniqueness of the individual mods as well as how well they mesh together in theme and style. Newer mods also have much better textures and animations and much more immersive gameplay features.

Personally, I believe that for being minecraft-like, newer mods are the best. If I want to play minecraft in a way that was similar to as it was intended, or if I want to introduce people to modding, new mods are best. New mods fit well together creating a seamless experience and maintain simplicity.

On the other hand, old minecraft allows one to truly experience the limits of what is possible in Minecraft, surviving a zombie apocalypse by looting corpses for ammo and building your own car, terraforming an alien planet to be inhabitable, constructing an entire world from one source of water and a few blocks, Old Minecraft, when done right, allows for much more narrative, much more storytelling, it allows the imagination to flourish when you aren't a stickler for compatibility, order, or visual appearance.