Assuming the Matthew text is correct, the difference is significant. It is another confirmation that anyone who is mortal, including the Lord, stands in jeopardy every hour. (See 1 Cor. 15: 30.) He simply could not claim perfection while in mortality because mortality is a time of change, challenge and temptation. After all, He was tempted while mortal just as every human soul is tempted. (Heb. 4: 15.) Though He chose to give no heed to it, He was nevertheless tempted. (D&C 20: 22.)
While mortal He looked to the Father in all things. (John 5: 30.) After concluding His time in mortality, achieving the resurrection of the dead, He was given all power in heaven and on earth. (Matt. 28: 18.)
Therefore, if the Matthew text is correct, and the differences are accounted for in what we have just reviewed, then the admonition of Christ for our own perfection is not just an earthly endeavor. It is an invitation to follow Him and His Father into a loftier state, as well. (Abr. 3: 26.) One where the final realization will come only as we are able to endure greater glory than a mortal may possess. (Moses 1: 5.)
It is good we know this commandment is possible to accomplish. (1 Nephi 3: 7.) It is hard to conceive of following the Son in this way. Yet it is He who pronounced it, and He who has promised to share the throne of His Father with all who will come to Him. (Rev. 3: 21.)
I am not perfect, nor anything like it. I have seen Perfection, know what it is, and can confirm I am nothing like it.
A harmonious symmetry of light, majesty, holiness, glory and power are all around Him who is perfection. When I read the admonition to "be ye therefore perfect, even as I or your Father who is in Heaven is perfect" I can hardly grasp how that gulf between us could be bridged. I understand about the Lord's atonement. I have certainly been the beneficiary of it and will continue to be so. When I consider the infinite gulf between His and His Father's perfection, and my own imperfection, I am left completely stupefied at the idea it is even possible. Nevertheless, He gives no command which He does not provide means to obey. Therefore the means do exist.
When I hear from the casual observer of the LDS faith the stupidity about how we are going to "get exalted," I wonder at what the reaction will be when they finally realize how great the gulf separating us from that result is. I have some appreciation for what will be required, and know it will be eons before that end can be attained by any of us. It will not be magic. It will be through incremental improvement, being added upon, growing in light and truth, and perfectly natural in the process. Joseph Smith put it in these words in the King Follett Funeral Sermon: "When you climb up a ladder, you must begin at the bottom, and ascend step by step, until you arrive at the top; and so it is with the principles of the Gospel -- you must begin with the first, and go on until you learn all the principles of exaltation. But it will be a great while after you have passed through the veil before you will have learned them. It is not all to be comprehended in this world; it will be a great work to learn our salvation and exaltation even beyond the grave."
We are not left without warning about how great the gulf is we are to cover in this bridge we are to cross. Even now it seems the best use of our time would be to meditate on the things of God day and night. The revelations inform us that "Whatever principle of intelligence we attain unto in this life, it will rise with us in the resurrection. And if a person gains more knowledge and intelligence in this life through his diligence and obedience than another, he will have so much the advantage in the world to come." (D&C 130: 18-19.) Yet we seem collectively often pedestrians in a crowd milling aimlessly about presuming Christ will furnish us an easy time of it. His atonement removes from us all guilt and shame. But for perfection, we must acquire it bit by bit, grace for grace, line upon line, growing by accepting more until at last we have obtained what is needed. That will be our own doing. He provides the means, and His Father ordained the laws by which it can be done, and they provide us with free will and the capacity to choose, but we must choose. We must accept. We must press forward holding Their hands in order to arrive at last, after an infinitely long journey, in the courts of Heaven itself, fit to reside there.
Be ye therefore perfect. And start on that this moment. For you haven't another moment to spare.