In short, you can't completely ditch that Fire TV remote just yet. Check out myfor more info.
Finding 4K HDR stuff to watch is a bit more difficult than on Roku. On the Fire TV there are rows for "Prime 4K Ultra HD Amazon Original Series," "Prime 4K Ultra HD Movies," "Prime HDR" and "Rent or Buy -- 4K Ultra HD Movies" but they're all Amazon. Roku offers selections from Amazon, Vudu and FandangoNow too in its 4K spotlight app, which also highlights ones with HDR and includes videos from YouTube and Spectiv. Roku also breaks out 4K-capable apps, while Fire TV does not.
Unlike Apple TV 4K, neither Roku nor Fire TV support searches (voice or otherwise) like "Show me movies in 4K" or "TV shows in 4K."
Sometimes, 4K HDR actually does look better
Depending on your TV and the video in question, 4K and even high dynamic range may not provide much improvement in video quality compared to "regular" HD. But it's almost never worse, and often, especially on a good TV with certain content, significantly better.
I compared the 4K HDR capable Fire TV to the non-4K Fire TV Stick using a pair of excellent TVs, the 65-inch LG OLED65E7P and Sony XBR-65A1E, both of which use OLED display technology. With "Mindhunter" on Netflix it was almost impossible to tell between the two, and the differences I did notice were most likely due to the TVs' different calibrations and capabilities, not the source material. The same went for "Mad Dogs" on Amazon.
But when I queued up "Le Mans," an Amazon original documentary, the HDR images raced to the pole. With original sections shot in HDR (as opposed to the older film footage), the blues and reds on the cars, the bright orange uniforms of race officials and the clothing in the crowd looked more full and lush, highlights popped off the screen and the images had a depth and realism absent from the non-HDR version. The standard 1080p high-def image didn't look bad, by any means, but the HDR version was clearly superior.
In case you're wondering, image quality between the 4K Fire TV and the Roku Streaming Stick Plus was essentially identical, as expected. With "Le Mans" playing on both in 4K HDR, differences again seemed entirely based on the TVs' calibrations. For the record I've never seen any image quality differences worth mentioning between streamers that pull from the same sources.
It is worth mentioning that Movies Anywhere films from non-Amazon sources (Vudu, iTunes and Google Play) won't appear in HDR or 4K, even when played from the Your Library section on Fire TV. Roku doesn't have a huge advantage there, however, since its native Vudu and Google Play apps don't support HDR (yet), although people with large Vudu collections might want to go Roku since its app supports 4K and Dolby Atmos.
Speaking of Atmos, unlike Apple TV the Fire TV can support Dolby's best in-home audio format, but no apps on Fire TV actually support Atmos yet at the moment. Vudu's app on Roku offers Atmos however, so again people with large Vudu libraries (and an Atmos system) might want to go Roku.
Fire TV vs. Roku: A close call, but Roku wins
Overall plenty of people are sure to choose Fire TV over Roku, especially once Amazon begins blitzing visitors to its web site with promotional pricing over the holidays. And they'll probably be perfectly happy using it, especially if they're already enmeshed in Amazon's jungle of voice control and Prime membership. But I still like Roku's 4K stick better, with its cleaner interface, better search and neat extras like TV control from the remote. Both have their strengths, however, and it's tough to go wrong with either one.