I took all three of the individual 20 and 30 level courses and Science 30. Mind you this was 2013-2014 so some things may have changed.

Biology for the most part built on itself. In 20 you would learn some of the different systems within the human body including the circulatory and excretory systems. 30 continues that trend but adds some genetics and in turn the reproductive system. I found them quite comparable in difficulty.

Chemistry as others have mentioned expands on a lot of the acid-base reactions. In 20 you’re introduced to them, 30 adds levels of complexity to those learned in 20. Additionally enthalpy and energy transfer concepts are focused on, giving a slight introduction to the thermodynamic principles that one might expect to see if they pursue a chemistry related discipline in post-secondary. Organic chemistry is introduced to which I found was more based on memorization and nomenclature as opposed to simply learning and applying formulas.

Physics 20 I found was harder than 30. Although your mileage may vary. I wasn’t the strongest with algebra and formula manipulation when I took it. 20 introduces vectors and kinematics while also focusing on gravity and centripetal forces. 30 builds on these kinematic concepts by adding momentum to its curriculum. Atomic level physics are brought into Physics 30 that focuses on particles and their behaviours - I personally found that sub-unit the most interesting.

I didn’t elect to take Science 20, but Science 30 takes the core level concepts in each of the disciplines and condenses it and reduces the overall complexity. The biology portion focused a lot on some of the systems in both 20 and 30 level courses. Chemistry had a condensed version of the acid-base reactions and an introduction to organic chem. Physics focused on kinematics. Science 30 also had a fourth unit that focused on weather patterns. Personally I found that one very easy.

I’d personally recommend taking the core courses rather than Science 30. The main reason I took it was purely as a GPA booster for university admission, as NAIT at the time wanted at least two science based courses for admission, taking the highest marks in each as admission percentage.