I'm really confused in both of them As Firestick 4K Supports HDR10+ and Dolby Vision the confusion is getting more strong. Don't care about Dolby Atmos as I don't have high end sound system. Is HDR10+ and Dolby Vision really worthy the price? I haven't experienced them so wanna know your opinion Sorry for my bad English. Price in My country Firestick 4K - $80 Mi Box - $48
I always say go for firestick 4K instead of androidtv boxes (exception being the shield) unless you need Chromecast.
Is HDR10+ and Dolby Vision really worthy the price?
It is if you have a Premium TV that can output enough HDR Picture mode Brightness to really bring out light and dark Picture details during videos playback.
The quality of the TV is the key, not really the media player used.
In fact you will get worse HDR picture quality on cheap 4K HDR TV Vs the usual SDR video content. Full Array Local Dimming on a TV is a must for decent LCD LED 4K HDR TV. Otherwise forget about it.
Mi Box doesn't support dolby vision, the processor is from 2015 and completely outdated, the ram memory is the ancient ddr3, there's less choice of apps, apple tv+ is an example. These devices and their performance is not comparable.
What TV will you be connecting your device to? Peak brightness capability of the TV is important for HDR content performance especially Dolby Vision content. Without a high peak brightness it is likely that you won't notice a difference in quality between HDR10 and HDR10+ or Dolby Vision, if anything HDR10 will look better on a TV with lower peak brightness. For proper dynamic HDR playback the TV should have upwards of 1000 nits of peak brightness.
As far as content Netflix and Disney+ support HDR10 and Dolby Vision content. Prime Video supports HDR10, HDR10+ and Dolby Vision but there is limited Dolby Vision content.