Physical buttons with braille are probably a bit easier for blind users. You could have the elevator read out the label of each button on the screen as it was dragged over, or use voice recognition. However, since a decent number of lifts have braille labels, a blind user would expect to be able to run a hand over the buttons, guess roughly where there button should be, and read the braille label - and once they'd found that button, they could then remember its physical position (two buttons from the left, three from the top) for future use.

Physical buttons also require very little power when not in use, and their cost scales relatively well regardless of number of buttons - whereas a touchscreen panel three or four feet high for a very tall building would be pretty pricey. Touchscreens do have the advantage of no moving parts, so they're waterproof and have no internal wear and tear over time (assuming a sufficiently tough protective screen), but monitor illumination dims then breaks fairly quickly, and any protective plastic/glass shield over the touchscreen could get pretty scratched up.