12GB 1600Mhz or 8GB 1833Mhz?

16Gb of 1600/1866 would be ideal. Photoshop can use a lot of memory, although it depends on what exactly you will be doing, if it's just light editing to photos get 8GB of 1600/1866.

You won't notice much a difference past 1600mhz except in certain situations in real world usage.
 

Tradesman1

Legenda in Aeternum
With Photoshop/lightroom etc, having excess DRAM is not a drawback and higher freq DRAM is often extremely productive, if you work with these a lot (and depending on your hardware (which we don't know) I'd suggest 12 GB for a tri-channel rig (X58 based) or 16GB in 2 x8 if the mobo supports it, for a dual channel (most anything else except for X79)