This PDF file compares the features of the three available models of the Kill-A-Watt branded power meter.
A device’s power consumption can vary over time, so if you’re trying to understand power consumption over time for a device, you need a meter that can tell you total kW-h for the time it was plugged in. For your purposes, any other data (volts, amps, phase, etc.) is extra. You can also do the cost math yourself based on the price per kW-h listed on your electricity bill.
Basically, the cheapest meter that reports kW-h is the best value for you.