First, is yours a 45 degree diagonal or 90 degree? Most 45 degree diagonals in fact use prisms rather than mirrors, and (though there are exceptions), mirrors will have fewer aberrations.
If you have a mirror, you can live with it until you can meet others in your area and compare equipment. Seek out a local astronomy club. They are excellent resources.
Finally, it's not just a question of transmission (which may range from 85% to 98% or so), but of optical quality (mirror flatness) and mechanical quality (is the mirror properly aligned with the openings at each end of the diagonal? Does the thumbscrew hold your eyepiece well without scratching it)?
William Optics makes a nice diagonal of the 2-inch size--one version slides into a 2-inch focuser, and another screws onto the back of SCTs from Meade and Celestron. Your 8-inch scope won't have a full 2-inch unobstructed light path, but there's no harm in using the larger diagonal, and the William Optics diagonal is of very good quality at a much lower price than TeleVue's.