What MB, As Almost all with 4 slots are daul channel. Most tripple channel MB have 3, or 6 slots.
As for as 12 -> 16 Gigs ram, Very little performance difference, with the exception of a few select programs (Games not included). What will make a difference is if you end up running in single channel mode verse daul/tripple.
If by chance this is one of the few that used for slots so that three could run in tripple channel mode, or daul channel mode by populating either 2 or 4 sticks - I would opt for 4 sticks in dual.