from a reliability standpoint they're just the same.
functionally (i'm going to state the obvious here) the "HV" has a higher voltage ability but lower overall wattage
the pinouts on the unit itself are identical
the form is identical
the only real way to tell a difference is the "HV' model has a sticker that says "HV"
---------- Post added at 08:08 AM ---------- Previous post was at 08:06 AM ----------
for most car PC installs of this nature, i'd recommend the M2-ATX (non-HV)
the HV is really intended for higher voltage environments, or environments where the voltages fluctuate outside of the 6-24V range
your typical 12v car setup should never need to take advantage of what the HV offers. i'd choose the higher wattage ceiling