I usually use SVN for version controlling. Default application for showing differences in the svn is the typical diff application which existed in the Unix-Like operating systems. It is in the terminal and honestly saying, it's ugly and I can't work with it, So I decided to use FileMerge which is an standard app, lives in Mac OS. It's clean and graphical and shows differences beautifully, so I decided to use it for SVN diff.
Then, What I should do for this purpose!?
I googled and finally arrived at a thread in stackoverflow.com and It works like a charm, These are the steps to use FileMerge as svn diff default app.
First of all, we must have fmscripts installed.

$ svn co http://soft.vub.ac.be/svn-gen/bdefrain/fmscripts && cd fmscripts
$ sudo make

That's all, now we can use FileMerge in this way:

$ svn diff --diff-cmd fmdiff FILE1 FILE2

It's obvious that upper command must be in a directory that is under svn repository.

You can use aliases to use the command in an easier approach:

$ alias sfmdiff = svn diff --diff-cmd fmdiff FILE1 FILE2

Here you are, that's all. now you can use it.

$ sfmdiff FILE1 FILE2

This is the simplicity of Unix that Mac Os inherits it thanks to Darwin subsystem.
Unix is simplicity.