thats like asking which is better, squats or leg extensions. anytime you can do a compound movement that hits the core its far better than iso movements like leg curls. you can handle much more weight and it builds better functional strength. how many times in a day do you need to curl something up with your leg? nothing wrong with leg curls as a finisher if you the bbing type but as a main movement sldl, gms, racks, pull thrus, glute ham raises, reverse hypers are always a better alternative. very few PLers do leg curls and most of these guys have phat hammies.
