Two well known ways to get better image quality are to use a sharper lens, and to zoom in to fill the frame (as opposed to zooming out and cropping in post). Sometimes there is a give and take between these two.
I'm considering buying a slide copier that attaches to the front of your lens to let you take high quality digital images of your 35mm film slides and negatives. The product website says that a 100mm focal length (35mm equivalent) is ideal because it will fill the frame with the image of the slides/negatives.
I have a 1.5x crop factor on my camera, so that would be a 66mm focal length. That leaves me with a choice of two lenses: My 55-300mm zoom lens, or my 50mm fixed focal length lens. The 50mm lens is sharper, but I would have to crop the resulting image.
So how do I make an informed decision between the two? Is there some numerical representation of the sharpness of the lens that I could use? (If I had data on each lens?) Or is there some rough rule of thumb?