That’s like asking if it’d be harder to cut off the left hand or the right. Technically I’m right handed so that one’s worse but are we going to pretend that makes the other a good option? Neither is fun.
I’d personally say Arena Assault is better, if only because it’s not based entirely on a dead play style.
Both demand high scoring for a team, but Arena demands it in every match, compared to AA only needing it once at the outset. Clear that first round and it’s all shenanigans from there. Arena is terrible because it’s outdated. Scoring is based off SP and BST, and that system at least made some sense in the era of armors, but nowadays it’s all Duos and Legends with adjusted BST who can do whatever, and Rallies are more useless than ever thanks to how prevalent removal of buffs is as an effect. But we still have to run the same bad skills just to score well in Arena every single match. At least AA let’s me use whoever I want after one match of setup.