I ordered a Quarq power meter today. My 2009 CAAD9 is currently set up with a Garmin 705 head unit which has the cadence unit on the chainstay. My question is, do I leave that unit on my chainstay, or with the Quarq don't I need it anymore? I guess the quarq has cadence, but what about speed? Do I just rely on the Garmin for satellites for speed?

I would think you want to leave your GSC10 on so you will still get wheel rotation. Otherwise, you're speed data can get pretty noise when climbing at slow speeds or with intermittent loss of GPS signal

I have a garmin 800 and a powertap, cadence/speed sensor on my road bike. My mountain bike just has the mount for the garmin. when i compare the speed of the 2 bikes I dont really see a difference. the GPS seems to do a good job. If i was in your position, I would try both, look at the data then make your decision.

Keep it. You'll want it if/when you ride indoors, and it does help in poor reception situations. Long story, but for now I ride (a tandem) with a Cycleops Joule 2.0 next to an Edge 705; the Joule reads cadence from my Quarq and speed from a sensor on the front wheel, while the Garmin reads cadence from my Quarq and speed from GPS. The Joule is far more consistent when displaying speed. I don't have a way to successfully use a GSC10, so I don't, and I don't know if the Garmin can read just speed from a speed sensor.