The FN HAR and AK have the same basic stats, yes, but the FN comes with an integral laser sight and Gas Vent 2. These are good things to have, especially since you'll be using full auto to compensate for your lower dice pool a lot.
This only matters for "ordinary" attacks - like Jack says, surpressive fire is a nice option that doesn't need recoil compensation. But at some time you do want to actually shoot at people, so getting some recoil compensation (by, for example, Gas Vents) is a good diea. Of course, you can mod the AK to have those as well - but then you end up at about the same price point.
Assault Rifles are simply better weapons, but if you, as a mage, want a more concealable option, Machine Pistols (and, to a lesser extent, SMGs) offer that in a nice compact option, while keeping the spray-and-pray advantage. You'll do a lot less damage, of course.
Better yet, get both. The FN HAR (or AK if you go that way, or Ares Alpha if you want to risk it - it's functionally similar enough) can be your hard hitter when you're out in the field and need some non-magical firepower, with the MP/SMG as your concealed firearm in more civilized places.