I debated the exact same question last year, and after a lot of research I took my chances with the Quarq. I was using it with an ibike for a head unit, but recently ordered the Garmin 500. All I can say is how glad I am I went with the Quarq. It's been bomb-proof in the rain (survived racing in Belgium), easy to zero, very steady over various temperatures, and the battery is a huge plus. The battery died on me during a stage race, but I was able to replace it in literally 2 seconds, and still had power data for the next stage, a huge plus IMO.
I've never used an SRM, but lot's of teammates have them, and you can't really go wrong there. The system is very solid and shouldn't be an issue without the warranty, but if you do decide to go with the SRM, unless you really want SRAM cranks, find a better deal on a DA one or even the SRM crank arms.
So, while they're both going to be great systems, the fact that the Quarq is wireless, user replaceable battery, lighter (this is WW!), and gives you just as good data and durability, that's what I would go with. Hope that helps!
Perfect...it's what I am going with...Thanks!