For your purpose, I wouldn't be concerned about light transmission differences. Besides, any such differences will likely not be noticeable enough to matter.
Some diagonals will have smaller clear apertures than others. This is more common with some of the prism diagonals than it is with mirror diagonals; but sometimes the reverse can be true. This might be of greater concern than light transmission when using a 24mm (68 degree apparent field I presume) eyepiece -- where you'll likely want a more fully illuminated field.
The preference between mirror-reversed and correct-image diagonals, in my opinion, will depend on additional details that you've not mentioned. If you're not matching the star fields in the scope with star fields on a chart, then I see no real reason to prefer one over the other. But if you are matching star fields, then you'll want to match the field orientations of the diagonal to that of the charts you'll be using -- or, if using electronic charts, you'll want to toggle the chart orientation to match whichever diagonal you have in your scope.
Beware, field orientation will rotate when either type of diagonal is rotated. With the way I use a refractor, the diagonal gets rotated to whatever position is most comfortable for me, and this will depend on the height of the eyepiece (which will vary with the altitude of the object) and whether I'm sitting (usually the case for me) or standing -- a situation where it's more tiresome to make prolonged, and more detailed observations.
Another comfort issue is the choice between a 90 degree and a 45 degree diagonal. (Correct-image diagonals are available in either format.) For me, for astronomical observations, I find a 90 degree diagonal to be far more comfortable to use. For higher altitude objects, a 45 degree diagonal can end up being more awkward to look through -- for me.
To answer your other question: I usually prefer using a simple, 90 degree (not correct-image) diagonal. But I have, and have made use of, other diagonals for special projects that are outside the realm of this thread. Sometimes it can be a good thing to have access to a variety of different diagonals. So if you get one that you find doesn't work out for you, you might want to consider holding on to it. You never know what the future might hold.