What types of characteristics can make one resonance structure better than another?
To decide which is the major and minor resonance structure you have to look and consider 3 cases...
1) Full octet = Happy. Be sure that all the electronegative atoms have an octet.
2) Number of charges. The best one is the structure without any charges. It is okay to have one or two charges, but structures with more than two charges should be avoided!
3) Electronegativity. We want the negative charges on MORE electronegative atoms and positive charges on LESS electronegative atoms.