SRAM once again upsets the power meter market with new offerings from $220 for a crank based system.  Using their Quarq technology SRAM integrates the power meter in two locations; within the spindle or attached to the spider.  Both can be purchased as a complete crankset or as upgrade kits for existing cranksets.  The most cost effective is the Apex upgrade kit comprised of a single left crank arm and spindle for $220, but SRAM has two more offerings in Rival and Force models both under $350.  Off road upgrade kits XO Eagle start at $326.  All models require SRAM's DUB standard bottom bracket.  If you are not currently running DUB it will require a new bottom bracket which will add $40 or more to the bill.  Installation can be accomplished by a competent home mechanic, but many will opt for professional installation which again will also add to the total cost of upgrade.  In comparison a Stages Shimano crank arm can be installed with 2 bolt removal of the old crank arm; something even the most technically challenged should be able to perform.

Curiously SRAM does not offer a pedal based system.  With the acquisition of PowerTap some years ago it would have been logical that they use their pedal based technology from the P2 pedals to develop their own model, but this never came to fruition.  And the once dominant PowerTap brand died a slow and painful death.  With the more recent acquisition of the French pedal maker TIME, a pedal based system may still be on the table.  The advantages of pedal based system are ease of installation, easy portability (you can put them in a suitcase), and you may swap them from bike to bike negating the purchase of multiple power meters and systems.  Left/Right power measurement used to be an advantage as well but most major manufacturers offer dual sided models.  Whether or not you need it is a different story.

Presumably the spindle is the best place to locate a power meter as the weight closest to the center of the crank is the most negligible in terms of moving inertia.  But the weight of most power meters has been reduced to grams.  In terms of reliability and accuracy Quarq (SRAM) is a leader, and all of their models require a simple coin cell battery replacement approximately 1x per season.  

Perhaps most importantly SRAM controls their supply chain.  In comparison to companies such as Stages that laminate their power meter to another brand's crankarm, they are not beholden to supply chain issues or price increases from those brands.  For example, if Shimano increases the cost of a crankarm 15% these brands have no choice but to pass this on to the consumer.  Or they could stop selling them product completely.  In short it is a very precarious place to be.  This exposure is somewhat reduced by offering "factory install" to customers existing crank arm, but only if the customer is willing to go through the process of sending off their crankarm.  I am surprised that Stages has not manufactured their own crank arms, mitigating this vulnerability, instead of attempting to enter the saturated cycle computer and stationary bike markets.    

The newest frontier is the customer installed power meter in which a kit is purchased and the customer laminates the power meter to their crank arm themselves.  As power meters get smaller and cheaper this is more easily accomplished, and best of all it requires no changes in componentry.  We may see offerings in the future pushing the purchase price of a complete power meter under $200.