I use a Stages power meter that replaced the left crank arm. I like it a lot.
Stages now had dual power meters (reads both sides of the crank).
By using this option I can change my rear wheel to the trainer wheel and still have a power meter; whereas with a rear hub based power meter like Power Tap you would have to have two wheels that have power meters.
Stages has sales throughout the year to liquidate prior year's models.