DAC does the digital to analog conversion or improves it if youll have an outboard one.
buffers offloads the source device from its load for maximum efficiency.
both can improve the sound depending on the devices youll be using with them. to know which one has more effect, you really need to try it out.
in my case, id say the DAC improved clarity, detail, openess. the buffer improved the highs, mids, and the depth.