A beef steak is about 270 calories for 30g of protein, cooked lentils are about 22g for the same number of calories. I'm not researched enough in bioavailability to comment on that part.
For an 80kg man, the "recommended" protein intake is about 64g, the difference in calories is fairly minute (assuming you compare 100% meat vs 100% lentils). If you're training/weightlifting and following the advice of 2-3g per kg of bodyweight, you're almost certainly going to supplement your protein intake anyway. Protein supplements are 2-3x the density of red meat, and available in vegan options if that's your thing.
For an 80kg man, the "recommended" protein intake is about 64g, the difference in calories is fairly minute (assuming you compare 100% meat vs 100% lentils). If you're training/weightlifting and following the advice of 2-3g per kg of bodyweight, you're almost certainly going to supplement your protein intake anyway. Protein supplements are 2-3x the density of red meat, and available in vegan options if that's your thing.