Secondary Hard Drive
Since your Mac is from 2012, both the connectors are the same speed.
As I quote from this article:


  2012 Models - Fully support SATA III (6 Gb/s) in both the HDD bay and the Optical Bay. I'd recommend just leaving your HDD in place (more sound proof, less installing) and put your SSD in the optical bay.


Keep in mind that Macs that came with a superdrive are not able to install windows 10 via usb in bootcamp.You will have to install bootcamp with the cd drive still in the machine.
At least that was the only way for me (mbp early 2011)