There are probably a thousand great arguments against transubstantiation already. Here's one more for your repertoire. Sometimes folks in the Roman (or even in the Lutheran) communion make the argument that Jesus didn't say "this stands for my body," but rather "this is my body." It is true, of course, that he said one and not the other. Nevertheless, we have to allow him the use of metaphor. We can show his use of metaphor in the gospels, but here's another instance that's perhaps not so obvious.
2 Samuel 23:13-17
And three of the thirty chief went down, and came to David in the harvest time unto the cave of Adullam: and the troop of the Philistines pitched in the valley of Rephaim. And David was then in an hold, and the garrison of the Philistines was then in Bethlehem. And David longed, and said, Oh that one would give me drink of the water of the well of Bethlehem, which is by the gate! And the three mighty men brake through the host of the Philistines, and drew water out of the well of Bethlehem, that was by the gate, and took it, and brought it to David: nevertheless he would not drink thereof, but poured it out unto the LORD. And he said, "Be it far from me, O LORD, that I should do this: is not this the blood of the men that went in jeopardy of their lives?" therefore he would not drink it. These things did these three mighty men.
Will our Roman and Lutheran friends allow David this metaphor? Or will they insist that David thought that the water from the well at Bethlehem had been transubstantiated (or consubstantiated) into the blood of the three mighty men? Surely, they will allow David his metaphor. So why won't they allow the Son of David His metaphor?
And as they were eating, Jesus took bread, and blessed it, and brake it, and gave it to the disciples, and said, Take, eat; this is my body. And he took the cup, and gave thanks, and gave it to them, saying, Drink ye all of it; for this is my blood of the new testament, which is shed for many for the remission of sins.
And as they did eat, Jesus took bread, and blessed, and brake it, and gave to them, and said, Take, eat: this is my body. And he took the cup, and when he had given thanks, he gave it to them: and they all drank of it. And he said unto them, This is my blood of the new testament, which is shed for many.
1 Corinthians 11:23-26
For I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread: and when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. For as often as ye eat this bread, and drink this cup, ye do shew the Lord's death till he come.