I have an SRM (wireless Specialized) and Quarq (S975). They both have stable zero offsets and if there is any difference in power between them, I certainly can't tell.
Battery: SRM requires soldering in a new battery every 3-400hours, with the Quarq, you unscrew a cap and pop a new battery in, 10 second job.
Waking crank up: My SRM requires spinning the cranks forward to wake them up, with the Quarq, I can spin them backwards and they wake right up. It seems pretty minor but it is so much easier to zero the Quarq at the start of the ride than my SRM.
Zeroing while riding: My Garmin 800 autozeros with my SRM, it's always dead on and means you really don't have to zero them after you start riding. The Quarq requires backpedaling 4-5 times to zero while riding.
If I were to purchase another powermeter, it would most likely be a Quarq.