A treasure key can be used on any level of Prison of Elders after finishing it. 28, 32, 34, 35. I know that completing higher levels gives additional completion rewards, cores, etheric light, etc. Does it matter if I use a key on 28 vs. 35 for the chances to get better equipment? I'm looking for something like the VoG and CE loot tables that explained which legendary and exotics dropped where. Or, ideally, that it doesn't matter so I can turn them in on 28.
I don't know this for a fact, but to the best of my knowledge it doesn't matter, with one exception.
The loot table of the big chest is the same regardless of difficulty. The items you can get include Queen's weapons, and Fallen weapons (like what Variks sells in the Reef). The only exceptions to this are the following:
- The first time you open the big chest with a character, you will get an exotic item
- After Prison of Elders level 35, it can drop (this might be guaranteed, I'm not sure) a ship, shader, and/or emblem
I am saving 1 key for when I finish Prison of Elders on level 35. In addition, there is a chance that the Elder Cipher will drop when defeating the final boss on Prison of Elders level 34 or 35. One of the steps of this bounty requires the Queen's Cipher, which only drops from the big chest in the challenge modes (level 32, 34, and 35), so it is a good idea to keep a few keys around for that as well.
From what I have read and seen from experience, the big chest is the same in all PoE instances. The differences are in the smaller chests, as guaranteed items will drop even if you don't have a key in levels 32, 34 and 35.
Also, the first time you open the big chest, the exotic is not guaranteed but the chance of getting one is increased. I know for a fact since I did witness a friend not getting an exotic on his first opening.