The problem with the ethical argument - that meat production is cruel and vegetable production is not - is that it makes all kinds of assumptions about plant anatomy and behavior that may be wrong.
In fact, the growing field of plant behavior strongly suggests that those assumptions are wrong. That plants aren't unthinking, unfeeling at all, but every bit as alive and aware of their environment as animals. It's a really fascinating field of study.
They've found evidence of an analogous nerve system (that appears to send pain responses), evidence of non-genetic memory, surprisingly advanced communication (tobacco plants communicate using over 3000 chemical compounds that mean different things), environmental awareness (you can perform pavlov's experiment on pea plants using wind for the bell and sunlight for the food), kin recognition, and more.
It's a lot harder for us to recognize the signs of awareness, pain, even - maybe - intelligence of a sort, because plants are completely alien to us. They communicate with chemicals, move by growing, live at a different speed.
For example, most of us would immediately recognize and empathize with any animal screaming in pain or fear. Most of the animals that scream to communicate are social animals. The scream serves as a warning, "I'm hurt or afraid, there's danger, watch out!" Grass plants, when cut, release a chemical that cause other grass plants to raise their defenses. It serves pretty much the same biological purpose as a scream, "I'm hurt, there's danger, watch out!"
But we would never recognize the smell of fresh cut grass as a scream.