there is a difference in performance unless you modify the BIOS settings.
Basically the M2_1 will perform at x4 speed (should be the max) while the M2_2 will perform at x2 by default, and changing the configuration in BIOS might mean that you will have to let some SATA ports go.
Additionally, on my end at least (PRIME Z-370 A), the M2_1 slot is positioned under a passive cooling section (had to take it out for installment and place it back) where it drops its temperature by 10-20 deg. (c) as opposed to the M2_2 slot which is between the processor and the graphics card, without any passive cooling and between 2 components which tend to get real hot.
