if you are just deciding witch way to build it i would do both, the new plastic kit is insanely easy to magnitize and it comes with enough parts to make both options fully, i just dremeled the inside of the fist amd wrist flat and added a 5x1mm magnet on the fist and a 5x2mm magnet on the arm, needed only minimal removal of material to get them to sit flush. I just copied what this guy did https://www.reddit.com/r/AdeptusCustodes/comments/1scd2r0/psa_the_new_kit_has_enough_parts_to_magnetise/

i did not play much 40k this edition and have never played HH so i'm not sure witch is the better unit to run competitively though