Triumph: Actually, they really don't sap power when they aren't being called on to work any more than a normal differential does. They work by a difference in axle shaft speeds not a function of the input shaft speed.
"Better" depends on where you're using it. Best is having both (but the ability to turn off the traction control). For just driving around in the rain, traction control helps alot. For launching a car or off roading, limited slip is god (for rock crawling, look into lockers. For high end drag racing, look into spools)