There are a lot of ways to raise meat for human consumption with care, mindfulness, and free of cruelty. We often say that the animals we eat have a great day, every day, and then it ends with one very bad instant. True, the modern agricultural system is stacked against the average consumer, but we can't condemn meat eating as a whole. On a well-planned small farm or homestead it's actually quite easy to do.
Here's a better idea. Engineer meat from animal stem cells. More energy efficient and less cruel (although maybe less tasty). Plants take less energy but provide less energy to humans (meat is more power efficient). Think of plant and animal like 1.5V batteries vs. 3V batteries.
Well, all the power stored in your beef-battery was charged with plants ... There is some upper limit for the energy efficiency of meat, and it is below that of vegetables.
I think most meat has more calorie per kilogram than plants. If you could grow it without having to feed the plants to the animal it's a win-win, no?
Nope. It would require less farming to feed people than to feed animals and people to eat those animals. Energy loss is an order of magnitude. If you ate the plants instead of eating the animals eating the plants it would be more energy efficient.
Would it take less energy to grow meat (in lab) or grow plant?
I don't know anything about growing meat in the lab, but it doesn't sound very appealing and I've lost any desire to eat meat long ago so I don't really care either way. The theoretical potential to grow meat isn't in any way a convincing argument to make eating meat right this minute either non-cruel or energy efficient however.