Tesla, what makes the Kill-A-watt different is that it is a kilowatt-hour meter. That is, it keeps a cumulative total over time. This lets you get a real measure for things like fridges, which cycle on and off over time.
A data logger can do that too. The demand data you collect gives you a curve when it's plotted on a computer. The curve goes up when demand is high and goes down when demand is low.
The curve will show the watts used at any given time. But the area under the curve will be the total accumulation of watts over time. The software can calculate this for you.
It seems that both pieces of information are valuable. With a Kill-A-Watt meter, you only know total watt-hours accumulated. With a data logger you find total watt-hours accumulated and you can see when demand was high and when demand was low.
