How do tiers work with choice chests? They don't seem to indicate their tier so I often  find it hard to decide between a choice chest and a high tier one.

EDIT: Also, I just had a weapon chest just explode and give me nothing after I bought it.   What was that about?

Choice chests still have tiers, but there is no way to tell it's tier beyond taking a guess based on it's price. That's just one of their downsides beyond their price.

That other chest you mentioned probably came from a destroyed store ship. Some store items from a destroyed store ship will still be available as loot, in the form of malfunctioning chests that periodically spark when on the intermission screen. Those chests have a chance to just explode when opened.