Since I have a PM with cadence, I opted to go with Garmin's new speed sensor that straps to your wheel's hub. That way I'm still using a speed sensor for indoor rides (I use eMotion rollers) but cleaner than having the sensor strapped to my frame.

It depends on your goals...

"I want cadence in all situations"
Indoor, you'll need to get the cadence sensor for the CT unless you're buying a powertap, which will also do cadence
Outdoor, your new PM will transmit cadence

"I want accurate speed in all situations"
Indoor, the CT can do that
Outdoor, GPS speed is good enough (unless you're doing rolldown aero testing). A powertap will also get you speed that is more accurate than the SPD/CAD. Crank-based PMs won't get you speed to my knowledge.

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STAC Zero Trainer - Zero noise, zero tire contact, zero moving parts. Suffer in Silence starting fall 2016

I thought that the GPS took precedence over the speed/cadence sensor when both are active. Wasn't there something about that on DCRainMaker's site or somewhere else? Someone with a better memory correct me...

On my Garmin, the speed sensor overrides the GPS.

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Team Kiwami
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Are you certain? On the devices I have seen over the years huge speed sensor only kicked in when a gps signal was not available.

I'm curious if this is changes and, if so, how you know.

It was based upon personal experience with my 510, but you did make me think about it, which got me thinking :-)

I went to DCR's site and he states that speed sensor overrides the GPS. Here's the link:

http://www.dcrainmaker.com/...netless-sensors.html

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Team Kiwami
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My Quarq transmits cadence both indoors and out. I also don't see how the powertap can measure speed more accurately than a speed sensor, they both use a magnet to count wheel revolutions.

To answer the OP's question: I think it depends on how you capture data indoors. I ditched the speed/cadence center because Trainerroad gets speed data from the kickr (which is useless with erg mode) and cadence data from the quarq. If I was trying to record data with a Garmin it will not let you pair both a Kickr and Quarq at the same time so by proxy you cant get both speed and cadence without an addition sensor.

Good find Steffen. I'm pasting the pertinent lists for everyone's benefit.

Order of speed data priority:
1. Power meter sending torque and speed (e.g. PowerTap). In this case wheel speed is needed to compute power.
2. Speed sensor
3. GPS

Order of cadence data priority:
1. Power meter sending torque and cadence (e.g. Vector, SRM, Quarq)
2. Standalone cadence sensor (or combo speed/cadence sensor)
3. Power meter sending power values and cadence (e.g. Power2Max, Rotor)

nb. this is a different philosophy to what Garmin uses for running. The user generally gets to choose whether to take speed data from footpod or GPS as the priority. (I'm not sure how it is now that 920xt and HRM-run both also measure run cadence.)

it's not quite impossible - If you use TrainerRelay you can re-broadcast the data. You could pair it as a speed sensor, while taking power and cadence from the powermeter. All the main power meters do cadence these days. This would however prevent you from having a separate speed sensor for better data than GPS while on the road, as mine at least doesn't support multiple speed sensors.