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I think with diagonals, reflectivity is the most important factor optically. The wave-rating (1/8 wave, 1/10 wave etc) is not so important because the mirror is close to the eyepiece, so unlike light reflected off the primary or entering through the objective lens, the light from the mirror in the diagonal does not have far to travel before reaching the eye.
Apart from the optics, there is the build quality. I have the cheapest of the AOE 2" diagonals ($60 without a 2-to-1.25" adapter), and I use it with my ED80. Optically, I cannot fault it. AOE tell me it's the same as the standard Meade unit. What I don't like is the cheap chrome finish, which will scratch up more and more (a machined surface like on better quality eyepieces would be nice) as I have to rotate it in my focuser with the refractor on an EQ mount.
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