In normal use, neither material should leak light if the cartridge is well made and not damaged. The bigger practical difference is reliability: reusable plastic cartridges are often preferred because they’re designed for repeated loading, typically use screw-on caps, and are easier to open in the dark.

Metal cartridges can work fine, but they’re more vulnerable to dents, and snap-on caps may pop off if dropped. If a dent affects the rim/cap area, or the cap loosens, light leaks become more likely. That makes metal a bit riskier in everyday handling, even if it seals well when new.

As for radiation/x-rays, any extra shielding from metal vs. plastic is negligible compared with the scanner itself, so it’s not a meaningful reason to choose metal.

Bottom line: both can be light-tight, but plastic reloadable cartridges are generally the safer, more convenient choice for bulk loading and repeated use.