Recommendation on laptop with two disk

I have an old laptop with a 128GB m2 sata and a 1TB sata HDD.

I’m going to upgrade the m2 sata to 1TB.

What should I do? Also upgrade the sata HDD to 1TB SSD and make a ZFS mirror? Or use the m2 sata as a OS pool and the HDD as a data pool?

Thanks for your insight.

Mirror. Because it’s a LOT nicer seeing checksum errors get automatically corrected, rather than seeing checksum errors be uncorrectable and require restoration from backup to resolve.