The M2 is a separate board you have to mount in your case which I didn't really have room for in my case. I believe the only difference between the two styles is one runs up to 24 volts and the HV goes to 32 volts. If you have a 12volt system you want the NON HV version. If you are running a 24 volt system (such as in semi's and some other vehicles) You will want the HV version.
My M3 supply has been working fine for me. I have no complaints. I have not tried it in my vehicle yet but so far it tests out great. I am using a 55 watt CPU and an SSD in my system using the onboard video.