If not planning to upgrade for an extended period of time, I say go with the Ph II X6. As programs become more multi-threaded, the X6 I think would have more longevity than the i5 750. Either motherboard should be fine. I prefer gigabyte over Asus. My luck with asus has been pretty poor.
One reason the price is so close is that the performance is about that close as well. Example: Core i5-750 vs Phenom II X6 1055T benchmarks
If you can pinpoint what type of activity is going to be most important to you - get the CPU that best supports that.
An example would be gaming - it looks like the i5 750 has an edge there. If you're more into HD video and technical adventures like running virtual machines I think the X6 1055T has advantages there.
Bottom line is that both overclock nicely and either one will make a good choice.
The deciding factor might end up being the feature set of the motherboard you choose. And the two boards you've listed look like excellent choices and match up pretty much feature for feature.