After reading the spec sheet for both, I'd say it's a tossup. The Streambar adds what should be higher quality audio over the standard TV audio, and single remote functionality for controlling the Roku and audio together. Although with CEC functionality the Ultra can also control the audio of either a TV or AVR.
However, the new Ultra introduces to items that no other Roku has, which is AC-4 audio support (it's an improvement over AC-3 and supports Atmos, but it's still lossy) and Dolby Vision. At the moment AC-4 isn't really used by much of anything, but it is one of the approved audio codecs for the upcoming ATSC 3.0 Digital OTA Television that is beginning to roll out. Since it supports Atmos, it's possible that some online services might be getting ready to use it as well. But I've seen nothing official about that. But if you have an existing Ultra, and those two items are not high on your need list, it doesn't appear to be an upgrade for the remaining functionality.
So, if Dolby Vision isn't a vital item for you, there's very little difference on paper between the Streambar and the Ultra when it comes to the Roku OS. The main difference is the Streambar having a built-in quality audio system, and that might be far more important for many users.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.