Japanese. Mainly because they do their research really well about a lot of words that are foreign to them and the emotional highs and lows in dialogues actually hit the most for me in JP.

I tried playing it in English many a times but just quickly changed it back to JP because you just don't get that emotional punch in EN, it's almost like the VAs are holding back on letting out their emotional energy.

maybe it's due to changing VA directors time to time but EN delivery most of the time sounds flat or smug and rude and the very opposite of the characters archetype.

Infact EN localisation as a whole can at a few times be misleading, many a times a character will say something in JP but in EN the sense/context and wording are absolutely different and thus completely change the value of the said dialogue and character associated with it.

I'd say things have definitely improved in later half of Inazuma and Sumeru and ofcourse because of some EN VAs who go out of their way to give their 101% like Arataki Itto's EN VA. :)