Obama-Clinton emphasized words such as "peace"; Bush-Reagan used "peace" less. Obama-Clinton emphasized "war" less; Bush-Reagan used "war" more. Obama-Clinton emphasized "freedom" less; Bush-Reagan used "freedom" more. Obama-Clinton de-emphasized "democracy"; Bush-Reagan used "democracy" more. Obama-Clinton emphasized "Islam"; Bush-Reagan de-emphasized "Islam."
By comparing their respective speeches, you elicit a clear contrast in the use of words and thus what concepts or ideas they emphasize.