I suspect a dedicated streamer will give you better sound than using a notebook, with or without a DDC (by which I assume you mean digital-digital converter). I don't know why -- people have ideas, but no one really knows -- but I've always heard better sound from a dedicated streamer.
Given a great source, I would not bet that I2S will be better than USB. I2S was an improvement over SPDIF in the early days of consumer DACs, but receivers in DACs have improved greatly since then, and USB arguably has improved the most. Also, USB has the advantage of being the interface in which the DAC controls the clock, rather than trying to follow the clock from the source.
So, you may not get improvement by switching interfaces, but I suspect you will get improvement by switching sources.
Given a great source, I would not bet that I2S will be better than USB. I2S was an improvement over SPDIF in the early days of consumer DACs, but receivers in DACs have improved greatly since then, and USB arguably has improved the most. Also, USB has the advantage of being the interface in which the DAC controls the clock, rather than trying to follow the clock from the source.
So, you may not get improvement by switching interfaces, but I suspect you will get improvement by switching sources.