Salvia and DMT are similar in terms of the speed and intensity of onset, and short duration of effects. For me, that's where the similarities end. Most people find Salvia so bizarre and not particularly enjoyable, it is not a substance that compels you to go back out of more than simple curiosity. DMT tends to be more enjoyable for most people - I've had degrees of ecstatic bliss with DMT that nothing else has ever rivaled.
One difference I find is that Salvia to me seems like a powerful "earth" energy, I often felt like I was being pushed sideways, stuck, or I would become inanimate objects. DMT for me was more "cosmic" energy - a feeling of moving up and out of my body into a bigger universe.
Salvia often destroys all your recollection of your life in this reality, or even that you have ingested a drug. It takes you somewhere else, and when you get there it's like waking up from a dream, and I often remembered nothing of where I came from, or who I was before the experience I was having. DMT is interesting because (for me anyway) the "ego" remained intact. By ego, I mean my sense of self, of who I am and where I came from. That was the curious thing about it, I still felt like "me" but my surroundings had completely changed. It's remarkably clear headed, in a way.
Chemically and pharmacologically speaking, they are completely different. DMT is an indole alkaloid effecting subtypes of serotonin receptors, whereas Salvinorin A is a diterpenoid effecting kappa opioid receptors.