I am moving to a new job in a Windows environment. Currently I have my Mac running to a Schiit Modi via USB and then from the DAC into a receiver. At my new job, I will have to output sound from either an iPhone 6s or an iPad Mini Retina (1st generation). Between these two devices, does one have a better onboard DAC?
The iPhone 6s will definitely deliver better results.
While I cannot speak of the iPhone 5's audio performance, there has obviously been an improvement between the iPhone 4 to 6 with the ability to play 24-bit audio. The iPhone 6's DAC measures very well up to the maximum 48kHz samplerate.
To compare it against the iPad mini, just read some user complains about Audio Quality.
Though, the iPhone 6 didn't play High-res sounds. If this changed to the 6s, I cannot tell. This article from mashable suggests to use the external Lightning-to-USB adapter to plug in your own DAC to enhance the audio quality.
What's a fair point to make is, when you connect an external DAC to your iPhone 6s, you will have with no doubt a supreme audio experience. I don't know with which headphones you will plug in, but I am assuming the hardware wouldn't be a downer, more so the MP3 or audio file itself.