Both Mormons and Jehovah’s Witnesses have been brainwashed thoroughly. The secret to this brainwashing is not only their different translations of the Bible, but the additional church sanctioned material that is propagated throughout their churches. Here is a segment from CARM that demonstrates this:
Consider this quote from The Watchtower magazine, August 15, 1981 that says:
"From time to time, there have arisen from among the ranks of Jehovah's people those, who, like the original Satan, have adopted an independent, faultfinding attitude…They say that it is sufficient to read the Bible exclusively, either alone or in small groups at home. But, strangely, through such 'Bible reading,' they have reverted right back to the apostate doctrines that commentaries by Christendom's clergy were teaching 100 years ago…"4
Did you get that? If you read the Bible by itself, you will become a Trinitarian because that is exactly what the Watchtower is referring to here when it says "apostate doctrines." In other words, if you read the Bible alone, you will not arrive at Watchtower doctrines.
Not only do these organizations enslave their members to their own church, but the doctrines are demonstrably written by those who falsely pronounce prophecies.
The Watchtower magazine for example has been published for nearly 100 years. In that time they have proclaimed a number of times that Jesus' return will be on a specific date (the last was in the 1970's). There is a good list (including citations) here. Obviously none of these are correct.
In Deuteronomy 18, the Lord gave Moses instructions regarding wayward and false prophets. He said to Moses "The prophet who presumes to speak a word in my name that I have not commanded him to speak or who speaks in the name of other gods that prophet shall die". This of course begs the question of how do we know if the prophet has spoken a word from the Lord or not? (Deut 18:20-21). The answer is given in Deut 18:22 – "when a prophet speaks in the name of the Lord, if the word does not come to pass or come true, that is a word that the Lord has not spoken…" In this case the people could be assured that the prophet could be put to death for speaking presumptuously. This is the way to identify a false prophet – what they say does not come true. Gods standard for a prophet is not 50%, it is 100%. Why? Because it is the words of God being conveyed. God gives those words and therefore he will not allow them to fail. Therefore from scripture, the authors of the Watch tower magazine are false prophets.
This is the message Jehovahs Witnesses and Mormons need to hear – that the chuch has enslaved them to a works based salvation when a works based salvation is merely religion that leads to condemnation.
If they stay in the church and reject the person and work of Jesus Christ, denying that he is indeed God incarnate, they will perish.
This must be our appeal – not an appeal to merely win a debate, but an appeal to them to repent and leave the ranks of the watch tower and turn to Christ alone for salvation through faith alone to be saved by grace alone for His glory alone. Unless they turn from the deception that enslaves them and repent, there is no hope for them.
Don't spend lots of time arguing with Arians. Demonstrate the truth about Christ from scripture clearly (use something like this) and give them a clear and simple instruction to leave the Mormon or JW church and repent and turn to Christ. Pray for their souls as you do this – there is no hope beyond the working of the Holy Spirit in their hearts to testify of the person and work of Jesus (John 15:26).
There is a distinction between mere religion and a true and saving life of worship to the one to whom worship is due. There is freedom in pure worship, there is nothing but slavery in religion. Unfortunately true worshippers are hard to find (John 4:23-24), even, sometimes, in evangelical churches.
If you have the opportunity to speak to a Jehovah’s Witness or a Mormon, take care to be concerned primarily with their soul rather than winning an argument. There is a great way to walk a Jehovah’s Witness to Christ from scripture here. Use it (or something like it) to lead them to Christ for salvation.