The Modi/Magni stack is cheaper than the E17+E9K and the Magni's a bit more capable or versatile as an amp (lower output impedance etc.). They're more focused/streamlined and less feature laden, you're paying strictly for the core DAC/amp functionality without bass boost options, portability, extra inputs, etc.
What's your budget? There's the ODAC/O2 too, slightly more expensive, not sure how the ODAC compares to the Modi... If you're trying to keep costs down I think you could easily opt for a $50 Xonar DG/DGX card and pair it with a Magni or O2 without missing much. (which would also get you Dolby Headphone for PC games)
In a pinch I believe you could even run the Mixamp as a DAC off the PC, tho it's far from the best solution. I believe the O2 has all the connections in the front so it might be friendlier if you're often gonna be switching the amp around from the Mixamp to the PC (whether to a DAC or internal card) and vice versa, the Fiio units would also give you more inputs.
So I see that there is the mini-USB connector on the back of the ODAC/02 combo and then a Line In on the front of it. So that means I'll be able to have the PC connected via the USB, and the Mixamp connected to the Line In?
Or would I be unplugging things every time I wanted to switch between PC use and console gaming with the Mixamp?