Hi, @Menta Elise
Thank you for posting in Microsoft Q&A forum.
Based on my experience, it doesn't matter which one be first.
But it's worth mentioning, to install SCCM 2409 as an update, we must have installed SCCM 2309 or later, we cannot upgrade from 2303 to 2409 directly, you may firstly upgrade from 2303 to 2309, and then upgrade to 2409.
To upgrade the OS of site system, you may follow this guidance:
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