GoodTimes I prefer to choose solutions that have a sustainable and predictable future and do not impose risks of vendor lock-in or abandonment. For me, OpenSearch is the clear winner between these two options.
I don't know on which information you're basing this conclusion on but to me it seems to be exactly the opposite.
Elasticsearch development is definitely more active and the product isn't going anywhere. OpenSearch is basically stuck at where it was forked 2 years ago, while Elasticsearch evolved quite a bit. Also, OpenSearch documentation isn't as good as the original, unfortunately.
Unless you're using the AWS managed service, which is the reason why OpenSearch exists, I don't see why anyone should choose a fork with less features, support and documentation...
OpenSearch GitHub activity:
Elasticsearch GitHub activity: