@practicallyuseful Yeah, me too. Before I visited, I didn't know that Hong Kong included Kowloon Peninsula and then the even bigger New Territories beyond that.
When Hebinuma says:
Things in Kowloon are the opposite from in Hong Kong, it seems [vol. 4, ch. 25, pg. 14]
Maybe it's a bit of a generalization, but it describes what I saw as the difference in feeling between Central and Kowloon.
Central (on Hong Kong Island) is the central business district (finance, entertainment, government); it and the nearby neighborhoods have some cool historical places (e.g. temples) and classic tea houses tucked between modern eateries, fashion stores, and art galleries. And of course the skyscrapers; the famous HK skyline is there (I would either go to the top of Victoria Peak via tram or bus or cross the bay to Kowloon via MTR, ferry, or bus to take in the full view). A smattering of ex-pats and foreigners, plenty of bars, more expensive. It's also smaller geographically, so probably more walkable.
Kowloon has a lot more traditional Chinese elements and older buildings, but the sheer amount of retail, markets, and eateries make it feel like the city center of Hong Kong. The bride and groom's families lived here or else a bit into the New Territories. Feels very much like where the locals would hang out. More "authentic"? Grittier? This is where I would want to live if I knew Cantonese (the local dialect).
That's my take based on my limited experience there and bits of information from my friend and their friends. Both areas were incredibly fun to visit. Not sure how the city has changed since. It certainly doesn't hurt to have a friend who is a local or at least speaks the language, but there's enough English posted everywhere that it's fairly easy to get around.