The answer to this is a sincere "neither."
If you want a pedal-based PM, wait for the PowerTap pedals. They may be vaporware at the moment, but I would trust PowerTap to get them to market a lot more than an unproven startup, say, Brim Brothers.
If you want a crank-based PM, wait for the PowerTap C1. It looks to be a Stages killer.
If you really need a PM before these products are available, buy a used PowerTap wheel or a used Quarq or something used and relatively cheap and sell it when you buy your P1s or C1.
Or buy something else that you like - the current generation Quarqs are good, same with Power2Max... PowerTap hubs... those are the cheaper options.
Personal disclosure: I don't own a single PowerTap product, I am on Pioneer PMs at the moment. But hard to argue that PowerTap hasn't really shaken things up and the C1 sure looks like a Stages killer (or more likely, a Stages price dropper).
Amateur recreational hobbyist cyclist
https://www.strava.com/athletes/337152 https://vimeo.com/user11846099