A prism generally gives better light transmission and image quality, because it has fewer air-to-glass transitions than a mirror-based system. That’s one reason pentaprism viewfinders are usually preferred over pentamirror designs.

However, mirrors are much lighter, cheaper, and often easier to adapt geometrically, which is why many stereoscopic/3D camera rigs use mirrors in practice.

If you use mirrors, avoid ordinary household mirrors: they produce a double image because the reflective coating is behind the glass. You’d want front-surface (surface-silvered) mirrors instead, like those used in optical instruments. Alignment is also critical.

So the tradeoff is:

• prism: better optics and transmission
• mirrors: lower cost, lower weight, more flexible mechanically

For a DIY stereoscopic system, front-surface mirrors are often the practical choice. If maximum optical quality and brightness matter most, a prism is usually better.