From reading both, I deduced that Spivak learned from Courant, and wrote a more modern, more theoretical version. It doesn't hurt to have both of those, or whatever ones budget allows. Spivak is lots of fun and extremely well written. Courant is also very well written in a more classically scholarly and serious tone.
the difference in terms of content is that courant has a more applied flavor and spivak is completely pure. E.g. there are more differential equations in courant.
the theoretical tone is also a little different since courant was written in the 30's and 40's and uses concepts like "accumulation points" while spivak was written in the 60's and uses slicker "least upper bound" axioms.unfortunately the days when i bought courant for $6.75 are gone, and it sells new now for over $100. so i recommend getting instead a copy of the rewrite by fritz john, by courant and john, as almost he same, maybe mathematically a little better or at least more modern, and often much cheaper.
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Introduction to Calculus and Analysis, Vol. II/1 (Classics in Mathematics) (ISBN: 3540665692 / 3-540-66569-2)
Richard Courant; Fritz John
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I have replaced my original link to an international edition with this one costing $5 more, since I am philosophically opposed to international editions which to me are to some extent illegal or at least immoral when purchased in the USA.there are also free smsg books that have more rigor than most texts today:
http://www.eric.ed.gov/ERICWebPorta...RICExtSearch_SearchValue_0="Herriot+Sarah+T."