If installing 16GB of DDR3 1600, which would draw less power - 4x4GB @1.35V or 2x8GB @1.5V?
It is module-dependent, but to get you in the ballpark we're talking here: http://www.xbitlabs.com/articles/memory ... html#sect1
. Pappnaas' hunch was pretty much right, as the difference is there in that test with 2x2, but it was barely measurable.
As for this case, more sticks to me suggests more draw. The difference in mW per stick should not be dramatic, certainly not double, between 4 GB sticks and 8 GB from what few specs I've seen. Motherboards have in the past been more reliable with fewer slots loaded, and it's arguably easier to install 2 rather than 4 modules, as coolers overlapping memory slots is common (big, quiet coolers and compact ones being the norm here), so I would argue 2x8 is the way to go for convenience's sake, even if you can't go DDR3L or DDR3U in that case.
PS. If you REALLY want to find out what's what, Micron is one place where you can find actual spec sheets of modules with readings down to the µA. In the meantime I'd just google for more system power tests.