If I get a key I guess only the level 35 large chest is worth opening week in, week out...
This is my understanding too. I think that the big chest contains the same loot chances (random queens weapon drops etc) across 32/34/35. The 35 also contains the Ship/Shader/Emblem (make sure you have spare slots when opening this). We have opened 3x 35 big chests now and only ever had that combo drop once across all 3 chars.
So, I would say unless you have some kind of key laundering system then:
- Use a key on the big chest once per week on each character, ideally level 32+ PoE.
- If you are going to do 35, make sure you have a key.
Edit: Pretty much what everyone else said
