which drive to add my operating system to?

To fully use the SSD, boot and Windows and installed programs should both be on it.

Because of the size differences between the disks, the strategy to use depends on the amount of used space on the current disk:

• If the used space is less than, say, 450 GB, then just clone the disk to the SSD using a product that supports size differences (example AOMEI Backupper Standard)

• If the used space is a little more than this amount, try to offload enough data so it fits into the SSD

• If the used space is much more than that, replace the hard disk by the SSD in the computer, install Windows and your applications, then add the HDD as a second disk (clean it up when everything works to your satisfaction).