Darius X

These are pretty competitive, and to recommend one over the other would require understanding your usage. Also, what other tools you use: for instance, what do you use for Issue-tracking, or for build pipelines. In your case, since you are already using Bitbucket, the question would be: do you have any current pain-points? And, on the other hand, do you already use Atlassian's JIRA, where you'd benefit from the tight integration? So, though I would not recommend one over the other just in general,. But, if Bitbucket fulfills your current use-cases, then there seems to be little motivation to move.

What is your objective here? It sounds like this is not to solve a particular issue, but is about learning? If so, do you have a preference for things used on the front-end or the backend? That decision would typically lead down a JAVA-oriented path or a JavaScript-oriented path.

If you decide to learn JAVA, then Spring + Spring-boot is a very relevant framework. A competitor is Microprofile + Quarkus. The Quarkus part is newer, but gaining a lot of traction.