You must use the control source for the scripts that you use to create / modify stored procedures. You can use SVN or git for this. Then the difference in file versions becomes trivial, since the version control client will have a great tool.
If you just want to split two files, you can use WinMerge (for free), which is great, or even FC.EXE (a file comparison tool that comes with Windows) to compare two files.
( ), Red Source Source Control SVN git. Red Gate Source Control SSMS, .