I've really enjoyed the accuracy of my PowerTap. It's a little flaky sometimes, but there's great service behind it -- really incredible.

So, for the <$700 option, there's just no touching a used PowerTap.

With a bigger budget, I think I'd favor Quarq over SRM. The idea of being able to change my own batteries is attractive, and it sounds like you pretty much need to mail the SRM off every year for service.

My dream setup: Quarq + iBike iAero or even wireless PTap + iBike iAero. Mobile wind tunnel with accurate power. That iBike is easily my favorite ANT+ head unit.