Which is better...?

In my opinion, both are good at something else, i would say it depends on for example what event is currently running

Both are good but brynhild is pretty rare,so I would go for her

I would say depending on the event. Farming event put Karna, For boss battle if the boss is divine put Karna, If the Boss is Male Archer put Bryn since 90% of male Archer had Bryhilde Beloved trait.

Most of the time people wanna farm, so Karna.

For me I like using rarer servants that have been given love (ie leveled skills and around 80-90) so I’d use a friend with Bryn over Karna if I’m just looking for some fun. However, when events start I go for servants with CEs that would help me along with if their class can help me through the run. Though I’d use Bryn over Karna if everything were the same because I have Karna as well.