The word "toxic" is overloaded and there are at least two main types of "toxicity" - acute and chronic. Caffeine is indeed more acutely toxic than cannabis in the sense that one could die from taking waay too much caffeine at once, but no deaths from THC overdose have ever been observed. Caffeine however seems to be almost entirely non-toxic chronically. Aside from addiction, regular caffeine intake doesn't seem to harm one's body or mental functioning in any way. Frequent habitual cannabis use has more negative effects, such as impaired memory.