The only thing PostScript and Forth have in common is RPN. Other than that, they are very different in philosophy - Forth is very bit banging, close to the metal, while PostScript is much more symbol oriented and high level.
That's true, PostScript is much higher-level and feels like a stack-based LISP. But, saying they just have RPN in common makes it seem like a small choice about the syntax - instead of a whole stack-oriented approach, which affects everything.
Well, yes, the stack oriented approach does matter. But even there, there are big differences with Forth having a user accessible return stack, which is implicit in PostScript, while PostScript has an explicit dictionary stack, which exists only in a very primitive form in Forth.
