I've had a fair bit of experience with San Pedro and Peruvian torch, both in natural and extract form, and I've had three doses of synthetic mescaline hcl (500mg each time, the last time was with LSD).
I was personally surprised to find just how very different the experiences were to one another. I found the visions on the cactus to be much more intricate, beautiful, colourful and liquid. The cactus also felt much warmer, grounded, earthy and benign whereas the synthemesc felt more shallow and superficial, and less deep for me. The visions on the pure mescaline weren't as colourful for me, and behind closed eye lids while peaking, my friends and I found the visions kinda jumbled, messy and confusing, they weren't really colourful or beautiful and there were very few actual images or visions, more just visual patterning, certainly not breathtakingly profound! For me this marks a stark contrast to the cactus, where the trip seemed to unfold like a story, seeming to have some guided narrative. This wasn't at all as apparent on the synthetic mescaline. Also a few of my friends experienced states of anxiety and fear on the synthemesc (never had this with cacus), I didn't but I had had a lot more experience with cactus than them. The synthemesc also seemed to be a lot more affected by set and setting than the cactus which seemed in a way more solid and grounded and less influenced by outside factors. Euphoria was apparent in the right situation but wasn't by any means a guarantee...I recall it liberally over flowing with cactus. Pure mescaline also surprised me by being much more sedating than the cactus which I wasn't expecting (everyone tripping experienced this each time), I thought it may have been the other alkaloids in the cactus that produced this effect. But at 500mg hcl, one had little desire to move around, if they did their efforts would tended to be met with nausea and desire to chill out. I definitely find cactus more user friendly than synthemesc.
So while I only experienced this three times, it was very similar in feel each of those three times, even though the setting was different and the people I was journeying with each time were different. So in conclusion I wouldn't be in a hurry to fork out for synthesmesc, so it must be that the other alkaloids in the cactus bring something else to the party as the difference between them was profound, and in my experience the cactus is definitely superior for the reasons I've mentioned.