Tubes are a more modern, technical version of the belay plate. They have two equal sized holes, so you can switch from single to double rope belays with ease. However, the rope-bearing surface is smaller and thinner than a plate, and is tougher on the rope's sheath. The tubes with an accordion pattern in the metal tend to disperse heat faster than flat sided tubes or plates, so they don't get as hot during long rappels. Since tubes are shaped like an truncated pyramid, they hold the rope away from carabiner better than their plate cousins, allowing for greater lock off friction.
Tubes are adequate for rappelling, but they tend to lock up, providing a jerky descent.
The consistent curve provides for smooth paying out of the rope, especially if you need a fast feed for dicey clips. However they don't handle two rope belays, and they twist the rope during rappels more than any other device. Auto locking devices have gained popularity for sport and gym climbing. A cam inside the device grabs the rope when it moves with a sudden burst of speed, like in a fall situation. If your climbing partner likes to hang from the rope while he figures out moves, an auto locker allows you to hold him in place, without holding onto the rope.