I have a M8S box (about £35 from fleabay) and a fire stick, both work equally well however I prefer the fire stick and using an app like apps 2fire its easy to side load any android app (not all work well though this is where the M8S wins), use firestarter to customize your start screen. Also the fire stick can be powered from the usb on the tv meaning no external power needed.
Once set up they both run Kodi equally well but I just prefer the fire stick, See my thread for using Kodi VU+ PVR, its been real head scratcher.