Christians: Is God a “just” god, or a “merciful” god?

I'm not sure why myself, but they do seem to reject the notion. I can only guess that they like to make it one of god's "omni" characteristics. They like to consider god to be all knowing and all powerful and such, apparently they like to consider him perfectly just as well.

Well unjustness often is bad. And I'd argue that the particular form of injustice that we're talking about actually does fall into that category. I mentioned a courtroom analogy earlier. Consider that there probably have been actual child murderers who repent on their deathbed and thus go straight to heaven. A christian would presumably think that such a thing would be a-okay, but I would not.

Of course the system is bogus either way because hell cannot be a just punishment for anything. An infinite punishment for finite crimes. 

And I would argue that his alternative to justice is indeed unjust.

And that's fine... if they could go on to explain that reasoning. Which they can't because they claim god is beyond understanding. It's just a way of saying "I can't explain why I'm right, but I will still assert that I am."

But then if god is beyond human understanding, how can any human reasonably claim to know that he is either just or merciful?

I put it to you that if that argument is acceptable, I can equally claim that god is utterly unjust, utterly unmerciful, and utterly evil. I have just as much evidence, and the argument makes exactly as much logical sense, as the christian claim for the opposite position. You can't even cite any of god's actions to argue against me because I can just say "Well that appeared just and merciful to you, but it was actually an act of evil - you just don't understand how it is evil because you aren't god."