Fire TV 4K doesn't cost even half of Apple TV. So Fire TV 4K wins in terms of cost. Although both the devices support 5.1 (And Dolby Digital Plus, Dolby Atmos), Audio quality is far superior in Apple TV, a winner without a doubt. Picture quality is almost same on both, however, here too Apple TV has slight edge due to better brightness levels, especially for HDR content. Streaming Performance is excellent on both the devices, although Fire TV navigation speed tends to generally slow down in the long run.
PS: If you do plan on buying Apple TV 4K, go for the latest one, as the previous generation one has a poorly designed remote.