The short answer is SSD, and it has been for years. If you're still running a spinning hard drive as your primary drive, you're leaving performance on the table — and probably not saving much money doing it.
The case for SSD is closed
SSDs boot Windows in under 15 seconds, open applications instantly, and handle multitasking without the stuttering that HDDs produce under load. The price gap has narrowed to the point where a 1TB SATA SSD costs less than a professional installer's first hour. It's not a debate.
Where HDDs still make sense
Mass storage. If you're storing video archives, backups, or large datasets that you rarely access, HDDs offer significantly more terabytes per dollar. A 4TB HDD for $80 makes sense as a secondary drive for cold storage. As a primary drive, it doesn't.
NVMe vs SATA SSD
If your motherboard has an M.2 slot, use an NVMe SSD. The speed difference over SATA is real and noticeable in anything I/O intensive. For basic office use, SATA SSD is perfectly fine. For anything involving large file transfers, video editing, or virtualization — NVMe.
One thing HDD still does better
Long-term data retention in cold storage. SSDs can lose data if left unpowered for extended periods. For true archival storage, HDDs or purpose-built archival media remain the choice.