1600 is sufficient, for sure, and very affordable. IVB memory support actually stops @ 1600 MHz, so that is part of it, for sure.
2133 MHz is faster, but has very little actual performance added overall to the system, yet costs twice as much, if not more.
myself, I feel 2600 or 2666 is where we should be looking, I mean, a jump from 1333 to 2666 and doubling frequency is pretty cool, no?
However, it seems many people out there are complaining that 2400 MHz or higher is impossible for some CPUs. I am not sure why, myself, to be honest, as running 2666 MHz for me has been really really simple. Some boards, nah, they don't like it, but it has nothing to do with my on CPU, for sure.
SO don't stop @ 2133 MHz mem if ya wanna that be a bit flashy...go all out.
I will say though FullinFusion, a memeber here, bought these Corsair Dominator Platinum sticks after he saw mine...he's pretty happy with them, and I understand why.
1.5 V @ 2133 MHz 9-11-10-27, and room to clock is kinda nice! Pricey...sure, but cost doesn't matter to me. IF it did, then yeah, I'd be sticking to 1600 MHz.