I'll have a go here. I grew up in San Diego and frequented both. I'd have to say if you're looking for a family beach, go to La Jolla. While Carlsbad isn't bad by any means, it's just a long strip of sand. La Jolla is in a cove, slightly better protected and more restaurants and hotels around. It's also a marine preserve. La Jolla can get expensive, but if you know where to look you can still get great deals. La Jolla is more central (slightly closer to Sea World). You'll also have Scripps Aquarium just a stones throw from you as well, if you've got the time.