The 27256 EPROMs are not produced anymore and are getting harder to obtain.
However the AT28C256 is still available a lot.
Also more programmers support the AT28C256 but often not the 27256 (for example the TOP2005+ universal programmer).
And the AT28C256 EEPROM is electrically erasable, not needing a UV eraser anymore, so another argument for using the AT28C256.
But the 2 chips are not 100% pin compatible.
Here’s a simple manual of how to use an AT28C256 instead of a 27256: