The prolific abo-abo has a nice bit of Elisp that you will find useful if you like to use dired for your file operations and find yourself diffing a lot of files. His snippet allows you to mark two file in dired, press e, and call ediff on the two marked files.
The code tries to put the newest file second so that
ediff will display the changes in a logical way. That is, if you add “new stuff” to the original file, you want
ediff to show the change as an addition of “new stuff” rather than as a deletion of it.