IMHO, *much* better when *a cat* sleeps on you. It's softer, fluffier, cutier, its purring is much more pleasant, and to get a cat, you don't need to became pregnant and to give birth, ohoho! XP
I don't have kids of my on (yet) but one someone else's kid falls asleep in my arms, I'm almost giddy! It's like they trust me enough to fall asleep. That's the best feeling in the world for me.