I have a Crucial 120BX 100 SSD1and a crucial M.2 275MX300 SSD4in which one I should install my operating systemand why?!
The BX100 looks to have a slight edge on reading but the MX300 has a huge edge on writing:
That's largely down to the size of BX100 you have, it's too small for much parallelism in its writes.
Since your system drive will be doing more writing that your secondary drive, it makes sense to have the MX300 as system drive IMO.