I support both professionally. The last Apache licensed version of Elasticsearch was a very capable product and Opensearch inherits all of that. In the few years since the license change both products have evolved a little bit but the vast majority of those changes don't really matter to new users. Both products use the same core component, which is Apache Lucene; which powers all of the search features. If you are a new user, there is very little reason to prefer Elasticsearch over Opensearch. And this is confirmed by the fact that most of my new clients default to Opensearch.
The exception to this might be vector search, which is a relatively new feature that was implemented on both sides post fork. Lots of users want/need this. And both Elastic and Opensearch provide independent implementations with very similar feature sets. Both build on what Apache Lucene offers on this front. So there isn't a massive difference between the two. But I would give the advantage to Opensearch here since its implementation offers a bit more beyond just the Lucene support.
Kibana had a lot of closed source components before the fork already and Amazon fork of that is a bit more limited. But notably Amazon indeed re-implemented the security layer (before the fork actually), which on the Elastic side is a bit of a dumpster fire of complexity. In general I'm not impressed with the product from a usability point of view. Either on the Elastic or the Opensearch side. But the Elastic version is arguably a bit richer in features.
Some notable recent changes there include trying to introduce a new query language based on SQL and the whole fleet ecosystem (an agent based system) to get logging and other data into Elasticsearch. I don't think either is getting a lot of traction because of the licensing.