Orion is much more interesting in terms of UI and also in performance, but I have to say that tab management on Arc is superior, as it has the concept of "folders", not only nested tabs. Orion has something like this planned, but there's no clear date.
The solution you're proposing (kanban style) is intriguing and something I would find pretty useful. Might even make me switch to Orion back.
I switched from Orion mainly because Spaces don't share cookies and sessions with each other, meaning that I can have [Private][Work - General][Work - Client] and use for example Gmail in each of those with different accounts, without fear of switching to a personal/work one.
I also lost tabs in Orion when syncing multiple devices, which Arc has handled flawlessly. It's interesting, because they both use iCloud.