Frankly, I think it comes down mostly to personal preference / looks / weight.
Quarq has more time on the market, and a good reputation (I would say especially in Triathlon). They used to have some problems with zero-offset going off with temperature changes, but I don't hear many complaints about the Riken/Elsa. They also have a good reputation in supporting the units out there.
P2M is newer, but I am quite impressed with how fast they evolved. 1st gen had issues with temperature changes, but they implemented temperature compensation. They are not the lightest, but the new Type-S is not that heavy anymore. They've built a good reputation of being a "simply works" solution.
I used to have a Quarq Riken in my previous bike. I had one issue with it: it locked-up for no apparent reason, and I had to reset it. I never had any evidence that it was drifting or giving me wrong values.
For my new bike I chose P2M for some reasons:
- I like the looks of the new gen Rotor 3D+ cranks, and also the possibility to move it to another bike (it is essentially a BB386 crank, and I could use in any BB, except in a Trek/BB90)
- I like the fact that I can remove the PM and use a regular spider in case the PM dies and I have to send it back for repair
- The new Type-S really seems like an evolution from previous generations... most of concerns have been somewhat addressed (temperature compensation, weight, the recommendation to use a specific brand of batteries). The new battery cover uses a very clever solution to protect the internals from humidity (you need a screwdriver, which I don't carry with me on the road... but frankly, I also do not carry spare batteries... if the PM dies, I simply ride home)
- Nicolas from P2M is always around here and in Slowtwitch, and if you send him an e-mail using P2M webform, he will give honest, non-BS responses
I don't have many miles on my P2M. The values are consistent, but I cannot yet give a personal opinion about long-term reliability and consistency. The reputation is good.
(I am in no way affiliated with P2M, Quarq or any other PM manufacturer).
DC Rainmaker is one of the best resources for bike-related tech, he prepared a PM buers guide a few months ago: http://www.dcrainmaker.com/2013/09/powe ... ition.html