Outside of a few select workloads, "more" DDR3 is beneficial than "faster" DDR3.
So, I'd opt for 2x8GB @ 1600MHz.. The addition of dual-channel operation will be beneficial too.
BUT, that assumes you have a workload that can take advantage of >8GB? Gaming can, depending on the title, as can heavy multi-tasking etc... and of course, editing/rendering etc too.
If your workload won't take advantage of >8GB, then I'd just stick to the single 8GB module you have. While the addition of dual channel should give you a bit of a boost, I doubt it would be proportionate to the cost.