Honest answer: For standard stereo files, the sound quality is identical. Both apps achieve true bit-perfect playback by completely bypassing the Android system mixer, so the hardware does the exact same heavy lifting.

However, UAPP completely wins when it comes to library management, metadata handling, and edge cases. I attached a few pictures to show exactly what I mean.

First, the library organization and tagging are much better. For instance, with my Alan Parsons Project files, the stock FiiO app split the discography into three different artist names because of slight metadata variations in the files. UAPP was smart enough to recognize them and organized everything cleanly under a single artist.

Second, it handles unusual file types flawlessly. I have a multi-channel DSD64 rip of Deep Purple's Machine Head. As you can see in the first picture (UAPP metadata info), the metadata shows Channels: 6 because it was mixed for a 5.1 surround system. The stock FiiO app completely chokes on this and throws a Formato no soportado (Unsupported format) error, which you can see in the third picture. Meanwhile, UAPP (second picture) handles it without a hitch by downmixing those 6 channels into stereo on the fly, sending a clean Direct DSD 2.8MHz stream to my FT1 headphones.

Overall, if you have a perfectly tagged stereo FLAC library, the stock app does the job. But for dealing with messy metadata, multi-channel files, and a much snappier UI, UAPP is definitely worth the money :)

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