Bismuth is better for those of us who want dynamic tiling.
Unfortunately, the developer no longer wants to maintain it, and while it's still working through KDE gaining a tiling API (that doesn't do anything dynamically, also what Polonium uses) for the most part, who knows how long it'll last...
Bismuth was broken for a while on KDE 5.27, which opened space for Polonium to gain some adopters. There's a long thread about the future of Bismuth. Unless someone else takes the lead, it'll be unlikely it'll keep working reliably on Plasma 6 and Wayland, at least during this transition phase.
I'm using Bimuth window decoration theme. You can also toggle that setting for specific windows with Plasma in System Settings/Shortcuts/Kwin/Toggle title bar and window frame (or something like that, I use this in Spanish). You can also customize this configuration per application.