My opinion - "butt" means buns and cheeks. models "showing their butts" would be photos of backsides, side views of the rump, and buttcracks.

"ass" is the bullseye between the cheeks. Spread the buttcheeks to get to the asshole. Showing ass means bending over and showing it all - or throwing your legs behind your ankles and proudly displaying all with a smile!

Whenever I see text like "straight guy shows his hot ass" and I click only to find buns and cheeks, I get mad.