Hello John,
i would use 9.1.x ASA code, it is the latest ASA software, so it has fixed most (if not all) of the known bugs.
However, i advise you to check the release notes of the version you plan to deploy, and verify the fixed bugs , and the open caveats (if exisiting), and make sure that you pick 9.1.x version that targets the 5585X platform which SMP.
Hope this helps.
Mashal
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Mashal Shboul