Considering endings A and B are probably afterthoughts specifically for the people who didnt like Michael or Trevor. I would say SCREW BALANCING. Since WTF is wrong with a happy ending in this generation of "They think its artistic and bold so they keep making sad endings because they assume its original but everybody makes sad endings making them unoriginal" endings. I am glad there is no consequence in this choice as it is plain masochistic if you want ALL 3 endings to have a sad note. This is most likely the cannon ending anyway.
Getting into an old argument here, but......
....I would argue that the "super mega happy" ending you get with C isn't much of a reflection of what's gone before it. It's not particularly coherent.
Trevor is a deranged psychopath who kills and tortures an awful lot of innocent people (as well as even more less innocent types) over the course of the preceding 60-odd missions.
Michael is a career criminal who sold out his best friend to the Feds to save his own ass.
With Ending C, neither of them face any repercussions for any of that - none of their previous actions have any consequences, and the tension between them isn't resolved at all.
Coupled with the fact that there isn't a really good villain for them to unite against.....just four pretty mild antagonists.....and you're left with an ending that's thematically inelegant and deeply unsatisfying. To me at least.
The problem is that Endings A & B are so weakly written and nonsensical that they're even worse.
GTA5 was a much better game overall in my opinion, but GTA4's endings blew it completely out of the water.....