决策 is mostly used as noun and Predicate, while 决定 can be used both as noun and verb.
决策 as Predicate in Idioms:
when uses as Predicate it's short for 决定策略, which means decide the policies and strategies to apply
运筹决策(make decision through operations research)
决策千里(make strategic decision from thousand miles away)
决策 as noun:
重大决策(big and important decision)
决定 as noun
我做了个正确的决定(I made a right decision)
决定 as verb
我决定学习汉语(I decided to learn Chinese)
when use as noun they have very similar meaning, only 决策 stresses that the decision is calculated and important. while 决定 is neutral.
by the way, 策 means compiled Bamboo slip(竹简) you can google photos for it. In ancient times our ancestors wrote on 竹简. important things are recorded on 竹简. so 竹简 becomes the symbol of knowledge.