From publisher blurb:
Both games rate traits with dice and feature a game currency called Plot Points to affect the story and alter the outcome of dice rolls.
Classic Cortex sticks to an Attribute + Skill mechanic vs. fixed Difficulties, as well as positive Assets and negative Complications that perform a lot like advantages/drawbacks popular in other games. Damage is measured in Life Points, with Stun and Wounds being taken in combat. This is the system used in Serenity, upgraded for Battlestar Galactica, further upgraded for a generic corebook as well as Demon Hunters and Supernatural.
With Cortex Plus, fixed difficulties are gone. All rolls are opposed. Core traits aren't necessarily Attribute + Skill; Smallville replaces them with Value + Relationship. Rather than continue to add up all dice in any roll, Cortex Plus has you take the two highest rolling dice and add them together. No trait goes above d12, there are no Life Points, and Plot Points are more potent and the flow of them is strengthened.
Smallville and Leverage take the basics and go in different directions in order to satisfy genre-appropriate outcomes. Smallville is supposed to encourage dramatic conflict between characters and centers on that; Leverage is about a team of experts who find their missions complicated by plot twists, but allows flashbacks to fill in the gaps and thus represent hypercompetence. Smallville forces you to take Stress and challenge what you believe in order to grow; Leverage instead allows you to call upon or invoke previous missions for in-game benefits and improvement.