
1. There are multiple variations of each model because of modifications. So there is no 1 size fits all differences.
Generally. The differences are:
MC-130J have buffer boards, HC do not
MC-130J have Variable Speed Drogue capabilities, HC do not
MC-130J have additional avionics computers and processors compared to HC
HC have specific rescue mods such as LARS.
MC-130J will eventually have terrain following radar, additional defensive systems, increased situational awareness and more advanced avionics compared to the HC. Years from now they will be vastly different after mods and will be easily distinguished.
Easiest way to spot an HC vs MC, right now, is the tail. HC will have a two letter tail code and tail number is big enough to be visible from a distance. For example moody is "FT". MC only have tail number and it's much smaller and not visible from a large distance.
Fresh off the factory from Lockheed, they are generally the same plane. The mods performed on HC by ACC and on MC by AFSOC are where they become different planes.