The Prime version is the English dubbed audio (which was recorded in Hong Kong for Toho) synced to SD video of the Japanese version. (The “Mushroom People” title originated with American International, which released basically the uncut Toho dub with new credits directly to TV. The title is presumably used on this streaming version because it’s more lurid or popular than “Matango”.) As with nearly every Toho export version, it’s basically an English language version of the original film. There’s no reason this upload shouldn’t be uncut.
I think the dubbing is OK for what it is. If you can tolerate how it sounds in the trailer I posted earlier in this thread then give the movie a shot, especially if you’re interested in it.
One thing worth mentioning: Media Blasters’ subtitles for the ending give a slightly different interpretation of what happens than the dubbed audio does. I don’t know which one is more accurate, although it’s probably the MB translation. I don’t think the difference is severe enough that it will affect your enjoyment of the film. It’s really hard to explain any further without spoiling the movie, so:
I hope you enjoy the movie, but regardless please let us know what you thought. The more discussion of non-Godzilla movies on this site, the better.