Reply To: Which OS?
If everything is working then I suggest staying with Yosemite until its support period ends. At that point you should definitely upgrade for security reasons.
If you upgrade now, some things could break, features that you depend on could be removed, etc. New features are sure to be added but it’s up to you if those features outweigh the risks (and costs). I couldn’t find any official support policy from Apple but they seem to support each OS X release for about 5 years. Therefore you could probably wait for a couple more releases before upgrading.