Answer: Neither.
Alternate answers: Both, or either, or split the difference. There's not much difference between 72,000 words and 72,674 words when it's an initial submission; and if they buy it and edit it, the length is going to change anyway.
0. Ignore your computer, your word processing software, and all such infernal machines. None of them do proper counts. You must instead visualize your manuscript as a document printed out on paper.
1. If you're using a proportionally-spaced font, pick out ten full-width lines of average density. (Don't pick out the longest lines; pick out full lines of average width.) Count the characters in each, including punctuation and the spaces between words. Add the results and divide by ten. If you're using a monospace font, just pick out a full-measure line and count the characters in it. This is your Characters Per Line, hereafter CPL.
2. Count the number of lines on a regular page. This is Lines Per Page, LPP.
3. Count the number of pages. Subtract one-half page for every chapter.
4. Now we adjust for more complicated formats:
4a. A text break counts as one line.
4b. If you have an indented prose excerpt, or a stanza of poetry or other line-for-line text, count one line of blank space at the beginning and end of the text, and count every line in that text as a full line. Thus, if your CPL is 74, "Tea for two / And two for tea / Me for you / And you for me" is 444 characters.
4c. If your book has charts, graphs, an occasional footnote or two, illustrations, sections formatted as scripts or screenplays, or other complications, don't try to incorporate them into your character count. Count the text as though it were plain prose. As for the rest, wait until the publisher has acquired your book, then tell your editor about it at some convenient moment early in the process.
5. Multiply CPL times LPP times the total number of manuscript pages (sans title page, dedication, acknowledgements, etc.) to get the total number of characters in the manuscript.
6. Divide that number by six. The result is your character count.
The three chief virtues of this system are that it's canonical; it doesn't vary arbitrarily between applications and platforms; and it usually comes out a bit longer than the other methods.