I caught Katie drifting to sleep while painting recently. This doesn't happen very often, but she is definitely more likely than Scott to fall asleep somewhere other than her bed. Often times at night she says, "I am SO SLEEPY" and she just wants you to carry her up to bed. Scott always has energy enough to keep playing, no matter what time the clock says.
If you ask Katie to sing a song, she will go into her squeaky "baby voice", but if you catch her singing to herself, then she goes into a fairly impressive full voice. We got a video example of each one -- including a very cute song she made up on the spot about grandmas and grandpas and great grandmas and great grandpas.