Fewer buttons, bigger ecosystem, or the reverse.
Button count is the most visible tradeoff
15 buttons versus 8 is nearly double the discrete triggers you can see and hit without paging through layers. If your workflow genuinely needs that many one-touch actions visible at once for raid macros, streaming scenes, and app shortcuts, the Stream Controller X's larger grid earns its bigger footprint.
Elgato's plugin marketplace is the quieter advantage
Software ecosystems compound over time: the more actively maintained and larger a plugin library is, the more likely someone has already built the integration you want. Elgato's marketplace has that edge over Loupedeck's, even though Loupedeck's own library still covers over 200 profiles and icon packs.
A swappable faceplate is cosmetic, not functional
The Stream Controller X's magnetic faceplate changes its look, not its function. It is useful if you want your setup to match a certain aesthetic and irrelevant if you don't care. If you're organizing a Discord-and-game setup like the ones in the OSRS setup hub, prioritize button count and software fit over the faceplate.