One character HAS to be more offensive and sarcastic. The balance in the Banjo games was just right! To be honest, I liked it the way it was and wouldn't invert the personalities. I think its best when you mainly play the good-natured, nice (but maybe slightly dumb) character and have his sidekick take the part of the foul-mouthed, intelligent but also sarcastic character. That way you would have a slight distance to his/her behavior and feel more like laughing about it. Another reason is that the main character starts the conversations in most cases, which wouldn't work as good when it always started with some offensive line. It's much better when the conversation starts calmly and get interrupted by some cheeky comments of the sidekick. This results in a better flow, I think!