I have studied the Peshitta for quite some time, and I am studying Syriac/Aramaic. And even though I do believe that most of the NT was originally written in Greek, I do hold the position that Matthew wrote his gospel orignally in Aramaic, and that is based upon the fact that the Fathers of the Church proclaimed that from the beginning with Papias to St. Jerome.
It is a fact that the Syriac has its roots that go back to the 1st century, and the Diatesseron is a testimony to that, which happens to be a work by Tatian. You have other very early versions that follow it such as the Sinaitic, Curetonion, Peshitta, etc etc. The Peshitta is much like the Vulgate to the Latin manuscripts.
But my point is this, Matthew wrote his gospel in Aramaic, and it was later translated into Greek, and the structure of the early Syriac shows that it is an independant tradition apart from the Greek as far as the gospel of Matthew goes, so it is of my belief that the Aramaic rendering of Matthew 16:17-18 is perserved the best as it is in the critical edition of the ancient Syriac, which is not in an agreement with a Protestant ‘reconstruction’ of the Aramaic in order to reject the Pope. The differnce between the Protestant ‘reconstruction’ of the Aramaic and the Aramaic that supports the Pope passage in Syriac is that the Protestants have no manuscript evidence to prove their case! But there are over 300 ancient Syriac/Aramaic manuscripts to back up the our argument!