SES for ARM should be used with the nRF5 SDK & all of the other standard SDKs, while the Nordic edition should primarily be used for the Nordic Connect SDK (NCS) in combination with the nRF91 SiP. However, you should be able to use the Nordic edition on the nRF5 SDK too (and the other SDKs). The main differences between the regular ARM & Nordic versions is that the Nordic version has extra features for NCS compatibility (i.e. setting zephyr base directory & opening up an NCS project).
