It really depends I must say but I'd go with this most of the time. Just like with every type of Story, it's in my preference to have a complete summary of the case at the end of the day at least. That being said, I enjoy the kind of setting that leaves the kind of uncertainty that allows people to think, theorize and draw their own ideas just as paying heed to those of others. If done well, that Mystery aspect can make a Story of this Genre quite more appealing. Adding a certain charm in things if you will !!!!

Both actually :P
However I usually prefer knowing the cause.

Both are neat, but I can get a bit antsy if plot elements remain too vague and open
But sometimes I really enjoy stories like that

I tend to prefer this simply because I feel like it's more realistic. I feel like if an actual apocalypse happened then you wouldn't know what happened.