Lore-wise? I don't understand the question?
If you're asking which weapon is more like the Corpus, probably the Ambassador (though you should probably just get a Supra or Dera if you want to cosplay Corpus).
If you're asking which weapon the Tenno would be more likely to have... I'm not sure that question has any lore with which to answer.
If you're asking which weapon would kill better in lore, the Arca Plasmor shoots plasma while the Ambassador shoots either explosives or electricity. Damage-wise my money is on the Arca Plasmor, because plasma is really hot goo that can chew through stuff while electricity can be diverted with a simple metal rod (and given the plasmor's AoE, that's probably enough plasma to make some sort of explosion of death wherever you shoot it). Practicality-wise, the Ambassador is more versatile and less likely to melt your face off while you're using it.
From a lore perspective the sisters of parvos kill lots of tenno, so I assume the Tenet Arca Plasmor is canonically a really strong weapon. I don't think we ever got any lore about the base form / ambassador.
Sisters of Parvos equipped with best of the best lore wise. So Tennet Arca Plasmor
