When searched for space repetition mode for emacs org-drill is the one google lists and looks popular but org-learn is written by the current maintainer of emacs John Wiegely. Its hard to choose one over another without knowing the pros and cons of each package.

 Those who have tried both please list the pros and cons of both the packages?.  

closed as primarily opinion-based by Dan Jun 4 '17 at 16:52

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