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.
^ Should be 6 per clk cycle. "Double"DR clks on both leading and lagging edge hense 2 x (comparded to single channel). data xfers: # sticks in parallel x 2 (3 x 2 = 6.)
However the advantages of trimple over daul channel in terms of performance is dependent on program and for most not an issue. Overall performance is limited by the "weakest" (slowest) link and in many cases only a benchmark will show the difference will the user may not even detect a differencein performance.