Hi!
Built quality is excelent in both case, the M11 still feels more solid (buttons) but also more heavy and big. The volume wheel on M11 is also more satsfying, personal mind though.
Overall use goes again to M11, with no lag in any situation (R6 lags a little while scrolling lists, for example). This being said, the availadity of Google services may be in favor of R6 if you care about them. Wifi on M11 is greate, did have no problem to download APK files to install Spotify, for example. I know the Wifi is not so great on R6, but keep in mind that I didn't notice any real concern on my own use, so if you want me to test specific tasks on M11 for the network strength, let me know!
The bigger screen is also a plus, with gesture navigation in addition, Fiio made a very close experience to a modern smartphone.
The Fiio M11 binds positive points of each Fiio X5 iii and R6 : the sound is really precise (Fiio) but also a bit emotionnal (R6). The R6 is not bad at all, but Fiio have the experience to gather you a very chirugical sound. M11 is still more neutral than R6, this way you may customize the sound as you want with specific EQ, headphones and keeping the best sound quality possible.
Let me know if you need more details on specific topic, but overall I think that M11 is superior.