All I can tell you is I can see NO visual difference between my Bintel 2" mirror diagonal and my Takahashi prism diagonal (Ooops. That was stupid - I'm trying to sell it!
Perhaps, way back when, when mirror-making technology was a bit hit-and-miss, a prism diagonal could be counted on to offer superior views.
Or I may be missing something and prism diagonals offer superior performance with certain types of scopes - but again I've tried both in TOTL refractors, Maks, SCTs and Newts, and if anything, been more pleased with the results from the mirror diags.
FWIW, the Bintel 2" (also sold as OPt, Istar, Astrotech etc etc.) is one of the best values in astronomy.
The Meade 2" IMHO, comes a close second.
The Stellarvue Deluxe Quartz 2" is better made than either, very solid and heavy, but optically about par. I reckon iots the perfect diagonal to help balance a big triplet, but a little heavy for a Mak or SCT.
If you can get one, the diagonals from Intes-Micro in Russia are guarenteed at 1/20 wave and actually are
noticably better than most others.
I don't think the uber-expensive digonals from AP, Televue, Takahashi or Baader give any noticably superior performance, so I'd stay away unless you need the brand cache.
But that's probably more info than you wanted!
Re your original question: Probably a good mirror digonal.