Wouldn't I want to make the most of this system and use a hexa-kit?
It's your choice. I'm cheap so I got the G.Skill dual kits. 24GB DDR3/1600 for $270 on sale.
Just so you know, according to page 2-11 of the Sabertooth manual, this is not a supported configuration.
"Due to Intel spec definition, X.M.P. DIMMs and DDR3-1600 are supported for one DIMM per channel only."
So even if you get the hex kit, it's not a sure thing.