Hey, it's Jason.

The VICE emulator is slightly faster than the MAME implementation. We actually got C64 emulation in MAME working at the Archive, but they're continually doing improvements and it tends to run at "only" 200% of native speed, which means that with the javascript/webassembly overhead it dips down below 100% in anything but the most beefy machines/browsers.

MAME is ALWAYS our go-to. But VICE is a particularly unique case, what with the quarter-century of development and the intensity of C64-only considerations they've brought in.