You'll want the airflow to go in
one direction only.
So the best idea is to have something like this:
<----<----<----(Fan blowing
outwards)
-----<----<----<-----(coolermaster) ---------
If you do this:
---->---->---->---->----(Fan blowing
inwards)
---->--||--<------(coolermaster)-----
The hot air is just getting moved around, and not out.
So to answer your question,
both blowing away is your best option
You can read more about it here:
http://blog.neweggbusiness.com/over-easy/pc-cooling-how-to-set-up-computer-case-fans/