[thx9000]

Hi, I'm hesitating between the Xiaomi Mi box S and Fire TV stick 4K. I have a Sony XF8305 and was wondering if the TV boxes have access to the inbuilt cable tuner at all. I've seen so many review videos and none of them shows an actual cable channel setup, only iptv.

We really do not know what you mean here.
No TV tuner hardware is accessible from any externally connected media player.

Plus I would not be using any of the Mi Boxes or the FireTV Stick 4K in combo with any sort of FTA interlaced TV content. Both have major problems with deinterlacing TV broadcasts. Avoid them.

Most IPTV is progressive video encoded content which any media player will be able to handle.   

I assumed that I would be able to watch cable tv and use apps using a single remote and without switching inputs. As laggy as Sony Android is, it's still better than constantly changing inputs. I already have my desktop pc with Kodi connected to my tv. I even have a shortcut that automatically switches from the monitor to the tv at Kodi's launch and back at Kodi's exit. With IPTV I see how it could be very convenient but then it usually comes with its dedicated box anyway. Thanks for the detailed answer, you saved me from a major disappointment AND some bucks there.