I would say the Gigabyte unless you need a mico-ATX board or an ROG board. They're both quality boards but the Gigabyte board has a few more features just because it's full-sized ATX. To me the Gene-z is a specialized board for those who want overclocking and SLI/CF in a micro-ATX form factor. For other users I think it's a little overpriced.
When buying any motherboard the thing you want to look for is how many x16 slots does it have incase you ever want to use multiple video cards , then you want to look for what features they have and how are they for overclocking. When it comes to overclocking all boards are not created equal. You want to be able to make a lot of different adjustments and not all MB do that. One company that is known for overclocking is Asus and thier Rog MB series are the best for OCing.
This is the same price as the Asus Maximus but is not a micro atx board so if you were wanting a full atx board you could look at this one.
However if you are ok with the limited amount of slots in the Maximus then that is an inexpensive way to get the Rog features that Asus offers and depending on what you are doing with your computer this would be a good choice.
It would have helped us more if you would of listed the components that you have and the main purpose of this computer , for example if you are not into overclocking then you could get the other Asus board in this post or the Gigabyte one. When you asked the question which one is the best it makes it hard if there is no other information.