Hello 4flush,
There are tons and tons of posts about this on the forum. You can try to search for CPA and learn all about it.
To give a short answer, CPA is mostly for when you want money instantly.
In the long run, a percentage is more profitable.
It also depends on the type of players you send. Those who play for a long time will surely be better if you take a percentage, you lose with CPA.
If your players are short term, then you are better off with CPA.
For some people combinations are an interesting way to do it.