Let me ask you one question?
Ferrari F430 or Or F136? Which one do you think is the best?
F136 is the engine that drives all the glory of Ferrari F430. One is not better than the other, because they are things of a different category, even though related.
What people don't realize is that Selenium is not a tool or a framework. It is an API, that you can use with your own framework, from open source frameworks like JUnit / TestNG (sticking to java based examples but not forgetting all other language bindings), or third-party frameworks like Katalon (yes - Katalon, and pretty much all other shiny frameworks uses Selenium to drive the browser - that's the only reason they are offered for free).
With the background, now trying to answer the question more directly. Which one is better is not a question that can be answered without the addendum of 'for what purpose'. If your target is to get a test bed set up quickly and you have no framework available, Katalon is your best bet. If your organization has a decent framework built over Selenium, go for that. If your target is to learn test automation, forget Katalon or Selenium - just focus on learning programming first.
Also, there are many other functional testing tools which serve a different purpose: https://www.simform.com/functional-testing-tools/