At certain points we say a "Yehi Ratzon." In my siddur Tefillat Hashalem, the construction is almost always "Yehi Ratzon Milfanecha." However, at the end of the Shmoneh Esrei for the end of Shabbat, the text reads "Yehi Ratzon L'fanecha."
My first question would then have to be, why the change? (I haven't seen this change in other siddurim but haven't looked through too many.) While the Talmud Bavli has many (about 30) examples with the Mem and the Yerushalmi has another 30+, the L'fanecha construction appears only once - in the Yerushalmi (and 18 times in Bar Ilan's corpus of post-Talmudic writing).
The Hadran in the back of the Schottenstein gemaras has one example of each.
Is there a difference in meaning? For that matter, what does it actually mean? The English I have seen renders both as "may it be your will" but that would just as easily be "Yehi r'tzoncha hashem elokeinu..." What does the word Milfanecha (or l'fanecha) actually add to that statement?