If Amazon Prime Video is your primary OTT subscription, you can undoubtedly get the Fire Stick. The experience you get on the Fire Stick for Prime Video isn't available on other Android TV devices.
However, if you like casting a lot of content from mobile and Prime Video is not your primary OTT subscription, and you like to install apps from Play Store and are familiar with the Google Ecosystem, you can go with MI TV Stick or any other Android TV. The new Google TV interface would be available across the Android TV devices in 2021 if that's something of your concern.
Another point to note is that the Dolby audio on Fire TV Stick works without any issues on all the OTT apps where it's available. On Android TV hybrid boxes like XStream and Binge+, this is a known issue.
Sideloading is available on both types of devices, so that should not be a problem. The performance also should be similar, unless you compare with the high-end Fire TV Stick.
