go for the 1600 its probably better atm for oc`ing and the 2133 tends to be unstable, hope that answers your question.
With SB, the speed of the memory doesn't improve the overclock. As Samx300 has stated, trying to run RAM at 2133MHz above 4.6GHz is hard. To be honest, trying to run it above 4.5GHz has been slightly challenging.
As far to actual improvements in your computing experience, RAM of that speed only plays a part when using things like Photoshop or 3D - rendering software. To improve game quality you don't need to go over 1600MHz unless of course you want bragging rights. Remember that increases to RAM speed increase the thermal output of the system and as a consequence, better cooling is required on occasion.