On Windows, I used a tool called
Araxis merge to merge and compare files. Unfortunately I have not been able to find anything as good on OSX but found something which just about does the job. Hidden within your Developer > Applications > Utilities folder on your Mac (after you have installed the Developer Tools that comes on the DVD with your Mac) is an application called FileMerge.app.
What you do is simply choose two files to compare and it will display the differences - you also have that option to merge bits too.
If anyone knows of something better - let us know!