CompareMyFiles is a great online service that lets you compare two text files and displays the difference by highlighting the text. In this way you can know which text file is more current, and get rid of the older one. It can also be used to check if someone has copied your work or not.
Here it how it works, select two different text files and click Compare Files. Make sure you select Skip Blank Lines, otherwise it will display blank lines as well, making the comparison more longer. It will take few seconds to upload and process the text files, the time would depend directly on the size of the file.
As you can see from the screenshot above, the difference between two text files is highlighted. It is dead simple to use. Enjoy!