M/K and K/M are two different animals entirely. One is meant to be a damage dealer, one is a full fledged tank. For M/K the damage dealer, I seem to recall a lot of complaints that the light damaging skills that you will get as elite skills aren't any better than regular mage skills. That being said, flame is your mages best friend. Keep that at a high level, and when you get fire mastery, make sure you master that as well. Fireball is nice as an instant, and lightning helps while you are questing to keep the big bads away from you. Wisdom, which logically increases your intelligence as opposed to the wisdom stat, is also useful, so try to keep that leveled. If you want to use the light skills, then put tp into the light mastery elite as well. As for the knight side, enhanced armor, resilience, holy strike, whirlwind shield, devotion, and maybe some into punishment are all important (punishment doesn't really help that much in endgame, but it makes questing a little quicker). And I know very little about the K/M elite skills. As a rule, level what you find useful, and level what you find fun.
EDIT
I realized I didn't really explain the advantages of one over the other very well. The K/M is a tank, capable of soaking in large amounts of damage and holding an enemy's attention, while the M/K is a dps, meaning it's job is to kill things as quickly as possible while not dieing. That being said, M/K is pretty good at not dieing since it has enhanced armor from the knight secondary, while the K/M really doesn't gain that much extra damaging power from the mage side. It all comes down to whether you want to be a dps or a tank, and that's 100% preference.
