Pre-2015, the general assumption this wiki (and most people, really) would use is that Hard Mode was canon since it features more story content, among other things. The problem is that there are a number of inconsistencies if one were to take Hard Mode as canon. In For Answer, for example, Ending 1 shows Lillium victorious over PQ -- this is impossible since PQ is killed by Strayed way earlier in Hard Mode. Another is Joshua O'Brien being "[Anatolia's] man" (according to Emil Gustav); taking Hard Mode as canon means that their so-called friend helped them only once ("Sol Dios") and in that mission, he's taken quick, which would negate the whole "together, Anatolia's merc and Joshua were unstoppable" (paraphrased).
People are free to choose what story they prefer, but I'd say that both modes are canon, sorta like how you can't take every For Answer League mission in one playthrough. The wiki should just use tabber to list separate Normal Mode and Hard Mode events (like in the biography section for Anatolia's Mercenary).