It seems to me the way I interpreted the specs sheet is that The ASUS Maximus IV Extreme-Z LGA 1155 will only run dual sli in x8. If you run Triple SLI it will run the first card x8,second x16 and the third x16? (Sorry I know that isn't a definitive answer, the specs sheet is not clear)
I don't think it has the option to run dual SLI in x16. (I could be wrong though, i was of the understanding that there are no sli boards for Sandybridge that have full x16 for SLi configuration, I thought it was coming with the "enthusiasts range" that is the LGA2011 chipset)
Also, when you say the other option is a X79 board are you referring to the LGA2011 motherboard that runs the 3930K and 3960K I7 extreme CPUs?
You might already be aware of this but if I am assuming correctly, the two motherboards you have listed will not run the same CPU's, that is, the ASUS Maximus IV Extreme-Z LGA 1155 will only be able to take the CPU's associated with LGA1155 chipset. (LGA1155 - I3,I5,I7 - Sandy Bridge) The X79 boards will only take the CPU's associated with the LGA2011 chipset. (at the moment that only includes LGA2011- 3930K and 3960K I7's).
If you decide to change motherboards you might need to look at a different CPU as well.