Both are just as good as each other in my opinion. Some people say that Asus's RMA service isn't very good and that Gigabyte's is better, but I have heard just as many people moan about Gigabyte as I have with Asus, so i'm not sure about that one.
I have had boards from both in the past and never had problems with either (apart from my P8P67 dieing) if you go with Gigabyte though, I would get wait and get one of their Z77 boards because they will all have a UEFI BIOS instead of the normal legacy one.
If it has to be between the Gigabyte GA-Z68XP-UD5 or Asus Maximus IV GENE-Z though i'd go with the UD5 because it's full ATX, all black and in my oppinion looks better than the Maximus.