a) Tell Camilla that both of them tried to cheat, and let her decide. Or we could ask her which one she likes more and then decide on whom to tell.

But my main issues are - since she's an intelligent woman, why should she immediately ignore one guy because of a letter? Surely the guy would go and ask her what's the matter and she would say that, and not just keep quiet about it? That way the guy would learn about the fake letter and another quest could appear, where he would want revenge on his rival, or that Camilla would suddenly detest the one who had tricked her and prefer the one whom she had disliked because of it? Or she could just laugh at both of them, that both try these tactics and decide according to her desires...
Although maybe the designers made it this way, that we choose whom she prefers, so that they don't have to choose if she really loves the Nord or the Altmer more...
What are you thoughts on this? I've just refused to end the quest because I want to tell her that both are willing to give her fake letters and don't want to stick with lying or telling her a partial truth. After all, both of them seem decent enough when you talk to her, the elf respects her for her intelligence and likes Skyrim, while the bard seems like a friendly, adventurous type, albeit a bit more self-assured than others.