I am using both Quarq and Stages PM’s on my mountain bikes. I’ve had good success with both. The Quarqs have been rock solid and I haven’t noticed the downward drift in power that some, including @Jonathan, have noted with some Quarq units.
My Stages meters are single arm and for the most part also worked really well. However one weakness is that they are prone to giving you some power spikes if you descend with the Stages crank arm foot forward.