Open two Internet browser windows. Go to the Spark Notes and Cliffs Notes websites.
Check each section separately. Start with Literature because that tends to be the biggest section on both sites.
Compare the list of books. The easiest way to do this is to print both screens and cross off the books that appear on both lists.
Look for the books you need to read. If Spark Notes has more of the books you need, use Spark Notes as your study guide.
Peruse the other subjects. Both Spark Notes and Cliffs Notes have sections for History, Math and Science. Cliffs Notes tends to have a better selection for these subjects. Cliffs Notes also covers books in foreign languages.
Use your own judgment. You should be able to tell if Spark Notes or Cliffs Notes will work better for you.