We manipulate the “Digital” control for pcm512x devices like the HiFiBerry/IQaudIO devices because we’ve been made to understand that this is the highest quality volume control on the device (I am not totally sure if it is a digital control or not. Many of the best DAC-integrated volume controls available today are digital–so digital volume control is not as much of a dirty word as it used to be).
As for Device Controls vs DSP Volume…I agree with @RBM. Both should sound very good. It’s really about personal preference and/or what works in your setup.