Ah someone who finally takes the NMM is a spirit route... I won't argue that it isn't her fault (I don't believe it was either/buy into Lauren Faust's headcanon + this is the creator of the show's headcanon, so it'll probably end up being confirmed in canon in S3).
Even if Luna wasn't responsible for NMM then the argument could be made that Celestia is the better princess by sheer merit of leading Equestria alone for a thousand years successfully (plus, the way she reacted to her sister was model behavior and inspiring). I wouldn't get into that because best Princess arguments are a bore, they're two very similar ponies and I wouldn't be surprised if in a parallel universe Luna did the same for Celestia. They're two awesome Princesses that are both deserving of your respect, admiration and appreciation. Tia and Lulu are best poni.
TLDR fans of both Princesses have the most fun.
I should mention though that as of now Lauren Faust's headcanon for NMM isn't confirmed, so you shouldn't say that with such certainty.
I don't really care about headcanons, especially that I already made up a very solid fanon based on some very interesting and convincing stories.
I can agree to disagree and say that both princesses are great. But from the two of them I prefer Luna's personality (and colors) more. In my eyes Luna was the bearer of Laughter, Loyalty and Honesty, while Celestia had Magic, Generosity and Kindness.