N.B. (Nota Bene) vs P.S. (Post Script)

Solution 1:

Nota bene is usually used in letter or documents to attract the reader attention to an important aspect or fact, postsctiptum usually refers to an additional note to what has already been written.

Nota bene:

• (imperative) Take special note; used to add an aside or warning to a text.

Usage notes:

• As with the Latin phrases id est and exempli gratia, nota bene is now more commonly encountered in its abbreviated form n.b.

• In Latin, notā is the singular present active imperative form of notō ‎(“I mark”, “I note”, “I observe”), whose plural equivalent is notāte; consequently,in English, when addressing an audience of more than one person, the plural form notate bene is occasionally used instead of the singular. This practice is not necessary in English;

• nota bene is regarded as correct usage irrespective of number by all but the most pedantic language users. The abbreviation n.b. may stand for either.

Postscriptum:

• Used to indicate a footnote at the end of a letter. Generally abbreviated PS or similar (ps, p.s., etc).

  • Addenda following an initial post script are often abbreviated PPS or similar (p.p.s, P.P.S, etc), indicating "post post scriptum".'

(Wiktionary)

Solution 2:

"Nota bene" means "mark well" or something to that effect -- the equivalent of saying "Important!". "Post script" simply means something written after the the main letter was written -- an afterthought. The two are the antithesis of each other.