MOBO: Gigabyte GA-990X-GAMING SLI or ASUS 970 PRO GAMING for overclocking?

Hey,
Looking at a specific value like the amount of power phases can be useful but in itself it won't tell you if it's a good choice to buy that particular board, and the overall design, parts quality and build quality also factor in. I would recommend looking at multiple REVIEWS instead to see how well a board overclocks.

Example: http://www.overclockers.com/asus-970-pro-gaming-aura-motherboard-review/

"The 970 Pro has enough power for decent overclocks and handled itself quite nicely in my opinion. Keep in mind this is a 125W board so even running the FX-9370 at stock is impressive, let alone at 5GHz"

So, it's not the BEST board in terms of overclocking potential, but keep in mind:

1) How much do you want to spend?
2) What about other features like SupremeFX audio? (better than a basic Realtek implementation). Some board like the Gigabyte have the M.2 SSD slot, and I think that Gigabyte has USB 3.1 Type A and C... I see the Gigabyte also has reasonably good amp and shielding for the Realtek chip.
3) silicon lottery (CPU may not overclock higher than the board is capable of)
4) PSU (VRM's etc on motherboard can clean up the power, but a poor PSU still may be the problem when overclocking)
5) CPU cooler (thermal limit)

So the above board is $105USD.

GIGABYTE->
https://www.amazon.com/Gigabyte-990FX-6Gbps-Motherboards-GA-990FX-Gaming/dp/B019C3S2CS

Just quickly checking the COMMENTS (and score). Seems like a fairly well made board, and reasonably priced. (Note that different online stores can have very different average review scores which seems odd to me, but don't compare one part at a store and a different part at a different store).

Gigabyte review:
http://www.kitguru.net/components/motherboard/luke-hill/gigabyte-990fx-gaming-am3-motherboard-review/10/

"Stability from the motherboard is good, thanks in large to a solid power delivery system that allowed for overclocking of the 125W TDP FX 8370 CPU. We managed to push the chip to 4.6GHz, with stability, using AMD’s Wraith air cooler. With enhanced cooling, the board was clearly capable of more than 4.6GHz (we actually hit 4.7GHz, but temperatures were high).
Speaking of overclocking stability, Gigabyte’s cooling solution for the motherboard is smart. The MOSFETs and 990FX chipset can output a significant amount of heat power when loaded, so the decision to use a trio of beefy heatsinks connected via two heatpipes is entirely justified and sensible."

Summary:
In general I prefer Asus, but I always look at the particular motherboard in question. That Gigabyte board looks pretty good for your usage.

I'm not going to look up and read enough reviews like I would if I was building my own. You can do that if you wish.

I suggest reading at least two for each board, and factor in what other FEATURES you want/need.

If in doubt, go for the Gigabyte board.

Other:
I'm not sure what you have now but:
a) there is no guarantee you will overclock much more than what you have now (so is it worth the cost?), and

b) I assume you have the computer built already? (If not, I would not be recommending AMD for CPU's)