They are all so unlike each other that such a comparison would be largely erroneous. However, I will say this.
DMT is, by far, the most overwhelmingly visual psychedelic I have ever used. It literally summons an entirely alien landscape in front of me for ~5 minutes or so, a dimension of impossible form and full of chattering, ancient intelligence. But it leaves my mind completely alone, my reactions are not magnified nor do I have any unusual perspectives. This is not to say that I am calm or detached during the experience, it is just that I am reacting in a similar way to how I would if hyperspace began spontaneously bounding through my kitchen window.
It is entirely different with the little mushrooms. They pour into the skull through the auditory system, a flood of tiny voices that then stimulate flowing visions. I notice a more human-concerned nature to the mushrooms, and a conversational quality in the visions/intimations that is not so strong with many other allies. What I am saying is, questions can be asked of the mushrooms, and answers will usually be given; whether the answers make sense at the time of revelation is not so certain.
The cactus-spirits are an emotional-visual-psychological whirlwind, very arresting and very wise. Giants among deities. They grab hold ones emotions, magnifying every thought and feeling tenfold. They reveal the knowledge of cacti, the sit-and-wait-and-grow-and-learn wisdom that comes with growing on mountaintops and in deserts for decades. The visions are electric, coming out of magnified thoughts and emotions; they are plasmoid and crystaline. These spirits do not let go. They stiiiick arouuund. By the end, fatigue can cause the visions to break down into the classic "mind-melt" symptoms. I would use more caution using mescaline or cacti than I would with DMT, ergolines other than LSD, or psilocybe spp., simply for the fatigue and longevity of experience for mescaline. I have not yet used ayahuasca, so I cannot say how cautious I would be of it.
Salvia Divinorum has a similar duration, but the effects are not comparable in any coherent way. They are really very very different, opposite ends of the spectrum. DMT is noon-sun-magic, Salvinorin alpha is full-moon-magic. For my psychic set-up, DMT's warming sun magic fit much better. Only a few days ago was I able to work positively with Salvia, after 3 largely terrifying and one difficult experience, though now that we trust each other I believe the little leaves will be a great ally.
Incomparable to any of the less-than-visual psychedelics that I have used, Ipomea spp./HBWR, A. muscaria, MDxx, DXM, PCP, or Sinicuichi.
