PostScript and TeX are designed for different purposes and for this reason one would never exchange one for the other as a replacement. However, from a purely theoretical point of view, both are Turing-complete languages and therefore anything that could be expressed in TeX could be also expressed in PostScript and vice versa.
That aside, PostScript is designed as a page description language and TeX as a typesetting system (or language). PostScript deals with lines, characters, rotation of a coordination system, scaling and colors whereas TeX deals with paragraphs, macros to define a markup language and generally provides many high-level services that PostScript hasn't built in: line breaking, hyphenation, breaking paragraphs into pages, footnotes, typesetting of math. Hence, PostScript is much more low level and indeed is one of the target languages for TeX engines: TeX is compiled to PostScript code (via dvips) or to PDF which can be considered a subset of PostScript.