>>96237981
the ion gun, left, has better armor piercing, better strength, (used to have better damage, rip) and fewer shots and shorter range. better for medium toughness targets like marines and terminators

the bolter, the gun on the right, has lower strength and lower armor penetration, but more shots, and longer range, and is better against lighter armor infantry.

that said, with the prevalence of leagues grudge token wound bonus, strength thresholds mattered to them less, so the bolter was often the preferred choice against all targets, but with them changing up that mechanic in the new codex, we'll see if that meta changes. this is also a unit that has not historically been relied upon to do any damage whatsoever, hearthkyn warriors are mostly there to claim an objective, die, and give your opponents grudge tokens in so doing, so it doesnt much matter what they're armed with, and their weapon profiles have been nerfed so heavily since their launch in 9th edition that its hardly worth discussing at length, and you should build the ones you think look the coolest because their impact on the game is virtually nil