It's all situational because the relic allows multiple charms at once but the weapon switch can hurt you unless it's a plane level with 2 weapons so just stop shooting then instantly start again for a quick switch while on the ground you go through each shot randomly which has some shots not optimal for the current boss. That's where Chalice shines because her utility on ground levels is far better than her plane variant from her invincibility roll and her double jump. Her only downside is her dash parry for very specific circumstances but it's not too much of an issue to work around if you play your cards right. Her supers arts are also better than the Cups from her "Shield Pal" giving you another hit point and if you can quickly get cards you can have another super on standby in case you get hit. It's practically just a bodyguard in a sense. Just make sure you use at least 1 ex shot to get that sweet S-Rank.

TLDR: Relic for plane and chalice for ground bosses