Otdfbt

Why are there two different versions of the Shabbat commandment in Exodus 20 and Deuteronomy 5? In Exodus 20 the Shabbat recalls the cosmogony in six days, but in Deuteronomy 5 the Shabbat memorializes the deliverance from slavery in Egypt.

י כִּי שֵׁשֶׁת-יָמִים עָשָׂה יְהוָה אֶת-הַשָּׁמַיִם וְאֶת-הָאָרֶץ, אֶת-הַיָּם וְאֶת-כָּל-אֲשֶׁר-בָּם, וַיָּנַח, בַּיּוֹם הַשְּׁבִיעִי; עַל-כֵּן, בֵּרַךְ יְהוָה אֶת-יוֹם הַשַּׁבָּת--וַיְקַדְּשֵׁהוּ. {ס (Exodus 20:10)

וְזָכַרְתָּ, כִּי עֶבֶד הָיִיתָ בְּאֶרֶץ מִצְרַיִם, וַיֹּצִאֲךָ יְהוָה אֱלֹהֶיךָ מִשָּׁם, בְּיָד חֲזָקָה וּבִזְרֹעַ נְטוּיָה; עַל-כֵּן, צִוְּךָ יְהוָה אֱלֹהֶיךָ, לַעֲשׂוֹת, אֶת-יוֹם הַשַּׁבָּת. (Deuteronomy 5:14)

More comprensively, [1]'The introductory statements for each of the versions of this commandment use unique language to describe the active commemoration of Shabbat - "to remember", on the one hand, and "to guard", on the other. [2] In both versions, the main body of the commandment consists of a similar list of laws, albeit more fully developed by presumably explanatory material in Devarim. [3] The conclusions drawn by each of the two versions seem to offer mutually exclusive philosophical underpinnings for the Shabbat.' (An article at Aish.com)

What were the original words spoken by God on Mount Sinai?

You can read in detail about the issue in this great article. To keep it short, Shevuot 20b mentions that the two versions were said in a single utterance, which is beyond human comprehension:

כדתניא זכור ושמור בדיבור אחד נאמרו, מה שאין יכול הפה לדבר, ומה שאין האוזן יכול לשמוע.‏

As it has been taught: Remember and keep were pronounced in a single utterance — an utterance which the mouth cannot utter, nor the ear hear.

^{Soncino translation}

Traditional commentaries learn from these two versions the positive and the negative commandments of the Shabbat. Another implication that can be learnt from the same page in the Talmud, is that women are obligated to fulfil these positive time-bound commandments related to the Shabbat (kiddush, havdalah).

Here's an explanation you might find satisfactory:

1. The first difference between the 2 versions is who said it - the first was said by G-d and the second by Moses and as indirect prophecy (it wasn't preceded by "וידבר הק אל משה לאמר" - he described the Decalogue in his own words.




2. The second is to whom they were said: the first was said to the Generation of the wilderness (דור המדבר), and the second to the Generation of the Land (דור הארץ).

Because the source and the audiences were different, the message targeted [slightly] different aspects of the Mitzvos, according to their abilities and predispositions.

The idea of the differences between the generations is explained in many sources but it is a subject on its own.