I'm gonna say...

It's a tradeoff...

Dark Knight Cecil looked more badass, but Paladin Cecil was total hotness.

Paladin Cecil, from a gameplay aspect, was more useful in almost every way... Almost... He had higher stats, more power, and white magic, but I sure did miss that incredibly useful darkness ability after he went through the change. It was almost useful enough to wish I could have kept him as a Dark Knight in spite of everything else. Almost...