I'm a big fan of power measurement for cycling, as it gives a no-excuses picture of how hard I'm really working. Heart rate and perceived effort are okay, but both are quite sensitive to environmental parameters and cannot be relied upon - even for a seasoned rider. I own two power meters: the Stages left crank arm, and the Limits meter that sits between the left crank arm and pedal. Neither have dual-sided capability, and this is where Team Zwatt's Zpider comes in!
There's no definitive answer to this. Most people will not gain any insight into running a power meter that measures both legs separately, as their legs are balanced in terms of physical output. However, you don't know how balanced your legs are until you do a dual meter test! For occasional checking you can pop down to your local gym or cycling clinic. However you will probably want to test each leg's power output on a regular basis, and in "real world" conditions. Most deal-sided power meters cost a lot of money, upwards of £800 ($1000), so the Zpider's refreshing price of $369 (until April 15, 2017) provides an opportunity to taste dual-leg power measurement without the massive cost. It is only available as a 172.5mm version, built around the FSA Gossamer crankset (compatible with BB86/30, PF30, BSA30, etc)
Already interested? Check out their Indigogo crowdfunding page: Team ZWATT Bike Power Meter.
The Zimanox is a cheaper option, as it is just a single-sided power meter built into the left crank arm. It will be available in a choice of lengths: 170mm, 172.5mm or 175mm. Its price is $249 until April 15.
Both the Zpider and Zimanox measure cadence, torque effectiveness and pedal smoothness. They are ANT+ and BLE compatible, feature a magnetic recharger (woo!) and should last around 200 hours between charges.
Team Zwatt keeps the initial price low by including a small monthly fee of around $5 and expecting you to upload at least one zwatt-measured 30-minute activity per month. You use Team Zwatt's own smartphone app (iOS or Android) to record the activity. At the same time you are free to use Strava, Garmin etc to record your ride.
The team has been building a decent picture of its power meter accuracy and reliability over the last three years. They indicate that it compares well with the Garmin Vector power meter pedals.
I can't wait to give my Zpider a try! Since it's BB30 compatible I will be able to switch it between my road bike and cyclocross bike without too much bother.
If you are interested in bike power metering, I'd encourage you to check out Team Zwatt's products. You can read about them at their Indigogo crowdfunding page: Team ZWATT Bike Power Meter.