In the actual world (per Christianity), we have the following scenario:
(A) God sends the faithful to Heaven instead of Hell, and he also sends Jesus to die on the cross.
Consider the following hypothetical variation on this:
(H) God sends the faithful to Heaven instead of Hell, but he does not send Jesus to suffer on the cross.
In (H), everyone has (as far as we know) the same amount of joy and suffering except Jesus, who has at least the same amount of joy but is spared a great deal of suffering, when compared to (A). So on (A), there appears to be an equal or lesser amount of total joy but greater total suffering than on (H).
If we take for granted that (H) is coherent, which certainly appears to be the case, then God, being omnipotent, has the power to instantiate (H). Instead, though, he chose to instantiate (A), thereby expressing his preference for it. So the Christian here faces a pretty steep challenge, to answer the following question:
(C) Why does God prefer (A) to (H)?
Now, we need to make the move from talking about the total balance of suffering and joy to speaking in terms of well-being, i.e. what is good for a person. It is not immediately clear how to connect the two, but it seems to me quite intuitive to suppose that, at least on the surface, (H) appears better for Jesus, and not any worse for anyone else. Maybe this is not the case, but it seems a reasonable initial assumption given the way (H) is constructed. In other words, at first blush, the following appears to be true:
(1) Everyone in scenario (H) has the same well-being as in scenario (A), except for Jesus, whose well-being is greater in (H) than in (A).
However it turns out that, despite appearances, (1) must be false, for the following reason: By hypothesis, God is omnibenevolent, which we shall take to mean (among other things) that he is principally concerned with the well-being of other conscious creatures. If (1) were true, then given this guiding principle, God would instantiate (H) instead of (A). However God has instantiated (A); so, by contrapositive, (1) is false.
These considerations set the context for evaluating the following possible answer to (C):
(2) God prefers (A) to (H) because (A) respects God's justice system whereas (H) does not.
Unfortunately, this answer is problematic. It's not that (2) is false; however it doesn't really explain very much, because it leaves a mystery why God should care about his justice system. It cannot be that God just really loves his favorite justice system for its own sake, since part of what it means to be omnibenevolent is to care principally for the well-being of others. Rather, God must love his justice system at least in part because it somehow results in (not necessarily strictly) greater well-being overall. So anyone using (2) as an answer to (C) therefore faces the following additional challenge:
(C') How is it that God's justice system results in a better total balance of well-being?
I submit that nobody has an answer for (C'). Furthermore, it seems clear that anyone using (2) to answer (C) hasn't provided a satisfactory explanation so long as (C') remains unanswered.
To see what we mean by "satisfactory," consider the following analogy: Suppose John asks Mary, "why did you go to college?" and Mary responds, "I went to college in order to obtain my college degree." Now, Mary's answer doesn't have to be false, exactly; indeed it seems quite natural to suppose that she went to college for the purpose of obtaining her college degree. Yet clearly she has not actually provided a satisfactory response in stating this fact. For John quite obviously wants to know why Mary cares about having a college degree, and in her answer she completely avoided dealing with that issue.
Similarly, (2) is not a satisfying answer to (C) unless we have an answer to (C'). Since we do not in fact have an answer to (C'), the Christian has not by offering (2) provided a satisfactory explanation for God's preference of (A) over (H).
In short, appealing to a system of rules to explain Jesus' suffering on the cross isn't helpful unless we understand how that system of rules improves our overall well-being. So, since we don't know how God's favorite system of rules of cosmic justice improves our overall well-being, it cannot adequately explain Jesus' suffering.