Help with a free build

I’ve got a PC I’ve built completely for free over the past couple years, and I’m finally getting some better storage for it. I still plan on keeping my HDD for bulk storage, but I’m going to be getting both a 480GB SATA SSD and a 256GB NVME SSD. Should I keep my OS on the SATA or the NVME drive? It doesn’t seem like the difference for it would be super noticeable for the OS, I’m more concerned about which drive I should use as the game drive to reduce the load times.