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I think you've got it backwards. As far as reflectivity is concerned, nothing beats the 100% total reflection a prism can offer. Prisms also have less light scatter than metal coated mirror surfaces.
All higher end SLR cameras (incl the ones you've mentioned) have pentaprisms, only the cheap models use pentamirrors. With the multiple internal reflections necessary to make a pentaprism/pentamirror work the mirror type will lose more light and result in a dimmer view finder image.
Prism diagonals lose light through reflection on the entry and exit surfaces, but modern multi-coating can bring that down to as little as 0.1% per surface IIRC.
Cheers
Steffen.
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