Justin,
Just read your question and suggest one thing in particular: if you're using this device for playback (only) then you don't need one of these audio interfaces. The two units you mention do both recording and playback. A pure DAC is less expensive and better for just playback.
A complete audio myth IMO, having owned fiio e7/9 combo and several audio devices under $300, I find the cheaper audiophile brands far less accurate. You get more bang for your buck with audio interfaces as they are used to encourage you to buy/upgrade the free DAW software. Most come with software like cubase protools ableton live demo versions hoping that you will later upgrade to the full version for another $3-500. This means almost zero profit is made on the interfaces, and they often have superior quality DAC's than budget/ebay DAC's. An interface has as little flavour or house sound, and aim to be almost flat in frequency response so "may" sound slightly different but its certainly not less detailed.
And just as importantly, the USB interface should be the newer "asynchronous" type. This effectively re-clocks the digital input signal from the computer to give you a much smoother sounding analog output. Neither of those units is new enough to have that so I suggest you keep looking!
Most audio interfaces will give far more detailed technical information, especially at this budget level, and when you consider they are designed for recording jitter and latency are highly important and considered as part of the design. RME devices use custom drivers (approved by MS) and have some of the best spec's you can get on any external device, even compared to $2000+ audiophile DAC's.
Simple answer is a interface will offer you just as good quality and more options, but you may pay a "slight" premium. Have a read of the ongoing thread regarding DAC's on the dedicated source components forums, its very interesting reading.
At the end of the day I would never spend more than 25% of my total budget on a DAC or interface other parts of the audio chain are FAR more important for playback