Assuming your dad is getting up in years, he would probably do better with the higher power. As people age their pupils don't open as wide as they did when they were young.
The size of the beam of light coming out of the eyepiece depends on the ratio of power to aperture in mm. A young person can use a ratio of about 7 to 1, but us older guys can only make use of about 4 to 1.
For example, 7x50 gives a ratio of about 7 to 1, good for a young person. But 7x35 (ratio of 5 to 1) is just as good for an older person because the extra light from the bigger lens can't get into the pupil.
So the 10x42 will probably appear just as light to you father as the 8x42, just more magnification. A younger person would see a brighter image with the 8x42. To get a brighter image he'd need something like a 12x60.
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