Could you give your Fatwa regarding the following: At the end of making Du'a is it permissible if the last words of Du'a are "Aameen" or it would be better to say "Allahu Akbar" as many people in my country do? To be more clear, these words are pronounced while wiping the face with hands, but I know (and tell others) that wiping the face is Bid'ah. So am I correct if I finish my Du'a with sending blessings on the Prophet Muhammad (Salla Llahu aleihi wa Sallam), then with the words "Rabbana Takabbal Minni Innaka Anta Sami'ul Aleem", then put my hands down (without wiping the face!) and say "Aameen".

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
"Allahu Akbar" is not a conclusion of the Du'a. In fact Du'a is concluded by saying: Aameen. Anybody who hears a person making a Du'a says: "Aameen" if he wants to participate in that Du'a and get its reward.
None of the pious ancestors is known to end up Du'a with the expression: "Allahu Akbar". Ibn Abbas interpreted the verse {Verily, the invocation of you both is accepted.}[10: 89] by saying that Musa was making the Du'a while Haroon was saying: "Aameen". This is why Allah said: "both" [Musa and Haroon].
So any person who says: "Aameen" after hearing a given Du'a he makes the Du'a equally because "Aameen" means: "O! Allah accept our Du'a and answer it".
Moreover, keeping on saying "Allahu Akbar" instead of Aameen is an innovation, which should be given up.
As for wiping the face with one's hands after making Du'a there is disagreement among scholars over its lawfulness. But, as a general rule the different opinions of scholars over a given rule should be respected because scholars have their own reasons for accepting or denying such a rule. As for this issue (wiping the face) it is narrated in a good Hadith that the Prophet used to do it. Al Tirmizi reported in a Hadith narrated by Umar Ibn Al Khattab that he said: "The Prophet used to wipe his face with both hands after making Du'a. He used not to put them down before wiping his face with them."
Abu Dawood and Ibn Majah narrated also from Ibn Abbas, that he said: "the Prophet said: "and when you finish (making the Du'a) wipe your face with both of them (hands)".
Ibn Hajar said in his book: Buloug Al Maram (Reaching what desired) after mentioning the above Hadith: "This Hadith has more evidence to back it, among which is Ibn Abbas Hadith which was reported by Abu Dawood and others. All these proofs make the above Hadith a good one".
So, all these are enough to support the lawfulness of wiping the face with both hands after finishing the Du'a.
On the other hand, it is likable to conclude the Du'a with invoking blessing on the Prophet as this is preferable and holds greater expectation that one's Du'a answered and accepted.
Al Tirmizi narrated from Umar that he (Umar) said: "Du'a hangs between earth and Heaven, nothing of it ascends until you invoke blessing on the Prophet".
It is also narrated from Ali Ibn Abu Taleb that he said: "Each and every Du'a stays blocked by a fence separating it from earth and Heaven until one invokes blessings on the Prophet. If one invokes blessings on the Prophet, the separating fence is removed and the Du'a is answered. But if one does not invoke blessings on the Prophet the Du'a is not answered".
Allah knows best.