Asus M4A89GTD Pro/USB3 or ASRock 890GX Extreme3?

There is no doubt. Go for ASUS. It's much better than AsRock.

Ok, but the ASRock card seemed to be faster than the Asus in some of the reviews.

The built quality and the stability of the mobo are aspects that should be taken in mind. The speed is not all. ASUS is a much better brand.
If you want a mobo, I recommend you to go for ASUS, GIGABYTE or EVGA.
If it is not possible, you can go, as a second option, for DFI, INTEL (intel cpus only) or MSI.
The only reason fo going for an ASRock mobo is the price.
Anyway, This is just my opinion.

Asrock uses solid Japanese capacitors.and is every bit high quality,or better brand snobs get me going , AMD loves the people that love flexible computing ,For another thing Intel are cowards as to RAM voltages,and speed limits.

I'd choose for ASRock 890GX Extreme3. It's far cheaper and best of all, it has overclocking and energy-saving feature, which making it stable motherboard.