Axial fans are better at pulling than pushing so they'll work better pulling air thru the rad. I tried mine in push and pull and picked up 1-3C with the fans pulling air thru. It doesn't make a huge difference and wont make or break your cooling but pull is better with axial fans.
As for replacement fans, the stockers offer the best blend of performance and noise. Anand did a comparison of a bunch of different fans and the ones that were better were louder and the ones that were quieter didn't cool as good so the stock fans are the best blend.
AMD FX-8150 @ 4.515 Asus Sabertooth 990FX Corsair H100
Corsair 500R Corsair TX750 Corsair Vengeance 2 x 4GB
Western Digital Caviar 320GB & 2x 1TB EVGA GTX670 FTW
BlackWidow Ultimate DeathAdder 2013 HP 2509B @ 1920x1080