Use "horrible" if it tastes bad or is prepared badly. Use "disgusting" if it looks like something that should not be eaten. For example, if it contained a dead mouse or a cockroach, that would be disgusting. Of the two, I would say "disgusting" is worse.

How about using 'inedible'?

When it comes to food, I'd say they're equally strong. They both describe food you'd probably spit out, or decline to try.

When used to describe food, disgusting is a stronger expression, but horrible also describes something very very bad, whether you use it for food or something else.