Given just those two choices I guess the Gigabyte board. If you are open to other suggestions, you might wan to look at a Soltek SL-75DRV5. Generally speaking, more adjustable features than you even need.
I'd say the 8K3A+ if you're looking for raw overclocking ability. It supports up to 2.3v (I'm a bit hesistant on this- is this right? Otherwise it's 1.85v) for the CPU and 3.2v for the RAM. The extra RAM voltage adjustment is my favorite feature (my Epox 8KHA+ has the same feature). It offers front side bus speeds over 200MHz/400DDR as well.
Thats what I need is one really good overclocking motherboard. my Soyo just isnt doing it for me anymore. It only has VCORE up to 1.85V and no memory adjustments whatsoever except for the usual cas timings, etc... No controling the voltage or upping the MHz though...
In time, I'll get a decent board...and get rid of this onboard audio crap, it just isnt doing my Z560's justice!