I currently have a garmin EDGE 305 with HR and CAD. I am buying an EDGE 705. If i get the PowerTap PRO+ Wheel Only will i still need the cadence sensor or will the cadence be picked up from the powertap?

The PT will send cadence data. It estimates it by sensing the pedal stroke through power surges. It does a pretty good job, but sometimes the display will lag behind a couple seconds if you are making a jump or changing cadence quickly. Not great if you are trying to do cadence drills and need to see absolutely real-time data or analyzing what was going on in a sprint second by second. I don't bother with my cadence sensor since getting the PT.

When my cadence sensor went out, my PT Pro+ did send my 705 my cadence. It was quite spotty, however, and it wasn't very steady. I would be pedaling at a pretty steady cadence and the readings would bounce around +-10 rpm.

I went and bought another speed/cadence sensor.

Just keep it. No reason to remove it if you already have it. If you had a Quarq or a crank based powermeter, you wouldn't need it because it's basically the same thing as it reads the cadence as part of the equation for calculating the power. But the Powertap doesn't do that, it is only an estimation.

I still use my cadence sensor because i was not satisfied with the cadence readings from my PT. It's not bad, but its not perfect like the garmin speed/cadence sensor.

the PT "real time" CAD number is very erratic. while the average it recorded seemed fine the high and low were not correct. this is due to the estimation of CAD of course. later.

This. I have a PT and a Cinqo and the PT isn't very reliable. I guess in the grand scheme if you're doing 85-90 RPM a few missed strokes isn't going to throw things radically off especially since the 705 will read data once a second at best but anytime you can have more reliable and hopefully more accurate data, it's a good thing. GL

Just one thing to be aware of with the Garmin GSC-10 cadence/speed sensor: it tops out at a max cadence of 180 rpm. It simply won't read more than that. If you can reach those RPMs, and care about the exact numbers, you'll need another sensor, because at those RPMs the PowerTap isn't accurate, either.

Not exactly an estimation, more of an extrapolation. It senses the change in torque in your pedal stroke, and calculates cadence from that.

I found cadence with the powertap to be reasonably accurate when you were putting out a fair amount of power. At lowe r power levels it tended to skip around a lot. So doing fast pedals (high cadence, easy gear) it wasn't so good for. Otherwise it was reasonably acurate.