Wich mobo is better for OC, ASUS M5A97 R2.0 or Gigabyte GA-970A-UD3P?

If you are changing BOTH the CPU and the Motherboard then why not go with an Intel setup?

The i5-4440 is reasonably priced and about 20% faster on average, though it varies between 0% and 40%. Skyrim, for example is about 40% faster on the i5.

The i5-4440 isn't overclockable, you'd need an i5-4670K for that, but very few games currently benefit from overclocking the i5-4440 (but you do need to overclock the FX-8350 in many games or it's a bottleneck).

I've been recommending this combo to several people:
http://pcpartpicker.com/part/intel-cpu-bx80646i54440 (or i5-4670K)
and
http://pcpartpicker.com/part/asus-motherboard-z87a