Dunuin is correct, the choice is in your hands and depends on your workload.
Personally, and without knowing your circumstances, I would mirror the SSDs using ZFS during installation of PVE onto the SSDs. Your installation will create two partitions, one of which is block based for VMs. You can set the max OS size during installation, something like 48GB or 64GB works for most cases. Limiting your swap may also be wise to reduce wear on your SSDs. As noted in other posts, for systems without much memory, consider limiting the ZFS arc cache (this will lead to a performance penalty but in my experience on SSDs you will not notice it compared to running out of memory).
Your HDDs can later be setup as a mirror and either be used for secondary VM disks (for large data/content) or backups.
There isn't a wrong way of doing this, but a mirror for both is my recommendation to increase your data resiliency. Also, be sure to check the disk wearout in the disks menu within your PVE node to monitor when your SSDs need replacing.
Cheers,
Tmanok