Genesis 1:6 ויאמר אלהים יהי רקיע בתוך המים ויהי מבדיל בין מים למים׃
(KJV) And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
(YLT) And God saith, 'Let an expanse be in the midst of the waters, and let it be separating between waters and waters.'
When God divides the light and the darkness in Genesis 1:4, notice that it is written בין האור ובין החשך. This same schema "ben X uben Y" is used throughout the scriptures when X and Y are being separated/distinguished. But Genesis 1:6 introduces another schema, "ben X la-Y." Are the two schemas equivalent in meaning? At first glance, they appear to be, but I'm not totally convinced.
The schema "ben X la-Y" seems like it could be directed, whereas "ben X uben Y" looks more symmetric. Perhaps "ben X la-Y" could mean distinguishing X from Y whereas "ben X uben Y" could mean distinguishing X and Y? I must admit, however, these would be virtually indentical in meaning.
If there is no difference, why would the Author of Genesis 1:6 prefer "ben mayim la-mayim" instead of "ben mayim uben mayim?"