These are some of the most popular power meters on the market today. All of them are built with strain gauges that measure power from either the hub, crank arm, chainring, or pedals. With some minor differences, the results from each of them is the same: MEASURING YOUR POWER OUTPUT.
You should do some research to see how each of them works. Read the review of their history in the market place but ultimately choose the one which would suit you the best due to convenience and/or price.
Visit their websites to see how each of them works, and their history in the market. Please feel free to contact me should you have any questions about them and which is right for you.