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-8350 @ 4.815 Asus Sabertooth 990FX rev 1 Corsair H100
Corsair 500R EVGA Supernova G2 750 Corsair Vengeance 8GB
Western Digital Caviar Blue 1TB x2 MSI R9 290x Lightning
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Corsair K70 Logitech G502