There are many summaries of scientific studies looking at the mescaline (and other active alkaloids) content in these different Trich species. They are not all in agreement, but seem close...many are online and you can just poke around the web and find them. Fulvilanus is reported to have only traces of mescaline...not up there at all. Terschekii is reported to be similar to San Pedro/pachanoi in alkaloid content, or maybe a little less.
The thing about Terschekii is that it is a huge plant, a suguaro substitute in Arizona but originally from Argentina. Suguaros are protected and their use is now licensed in some states, so Terschekii's are a good alternative. I've seen many specimans in nurseries than specialize in cactus, and are expensive, especially relative to San Pedro. San Pedros also grow faster.
The Terschekii's are beautiful plants - some can be quite stunning - and I would use them for lanscaping purposes...also VERY thorny. Make sure you have space for them. But psychonauting-wise, San Pedro would be a much better and far easier bang for the buck.
That said, each species has its own alkaloid profile. If you are into blending your varietals in order to obtain your personal perfectly balanced brew, then including some Terschekii could be a good thing. SWIM has heard (maybe in Erowid reports?) that it comes on fast and is strong and perhaps a bit "darker" ala bridgesii or scopulicola than straight pachanoi.
WHOA!