In terms of what's best career-wise, I think it would depend a lot on where you end up. As far as what's best for the community, obviously many redundant papers are not ideal, put but it would make sense to publish your result separately if both hold:
- it does not quite fit in with the rest of the other paper (as you already mentioned);
- you find it interesting enough that there is reason to believe it might be used later.
We like to complain about the over-abundance of papers in our fields, but, too often, very useful results end up in footnotes of almost unrelated papers, which makes it awkward for reference and dissemination. And there's nothing wrong with concise papers.
