From my partners experience, plugs tend to get used most for stretching purposed before pegging, or to provide a naughty, full sensation. The prostate stimulation is minimal, especially with smaller plugs for a couple of reasons: 1) Lots of plugs, smaller ones especially, are tapered, having a thinner tip for insertion, they are also short-ish, so the part hitting the prostate (with smaller plugs, is either none, of the thinner tip area. 2) Butt plugs are not really designed to be moved in and out.
With those things in mind, it is fair to say butt plugs, unless on the larger, longer side, tend to provide not much prostate stimulation and even the bigger ones that can, are only really adding a constant, stationary sensation.
So, I noticed very quickly that my partner got a lot more specific prostate stimulation from more specific prostate toys, toys designed to move back and forth easily (The texture grabs, rumbles, pulls or pushes at the prostate if the toy can move) second, a curved shaft toy allows extra pressure to hit that spot more deeply.
Therefore, if someone came to me and asked to recommend a prostate pleasure toy, I would most likely recommend something like the Aneros toys, or similar.
I guess the male prostate works in a similar way to the female g-spot, often needing more than simply a stationary object against it, for intense pleasure. I think butt plugs therefore provide a tease (but this all depends on the size, shape and preference of the man) I personally think butt plugs are better at serving other sensations, as previously mentioned, the stretching and filling sensations plus extra intensity during orgasm as your muscles squeeze around it and press it into the prostate (Less intense if the toy is small)
Anyway, I am female so I dont speak from utmost experience, just from my partner talking to me and reading a lot online. lol Good luck xx