Indoor training has begun already and I looking for some advice.
To get the most from my trainer training would you recommend a cadence sensor or a speed sensor for the rear wheel? (And why is it that Polar can make a cadence sensor that can transmit to a handlebar mounted watch, but not a rear mounted speed sensor? but I digress…)
I have a HRM, but think a little something extra to gauge my workout would be helpful.
Thanks,
DmJ
if you have to choose one, then speed.
I think that cadence is more important on the road and even more important on the trianer. IMHO speed is the least important thing that you can measure while riding. Speed is too effected by wind/grade/etc. on the road, and there is no way to accureately translate this to a trianer.
the best is obviously watts, but second to that I would run HRM and cadence to keep your form in line and make sure that you were trianing in the correct zone.
Personally I think knowing the speed doesn’t really matter on a trainer. Heart Rate, Cadence and working with the correct gear is more important and ultimately that is what dictates the speed anyways. So I would rather know my HR vs Cadence vs Gearing and what Power I get. Hence the computrainer that I own 
I would go with the Cadence sensor.
of course, if he keeps things consistent (tire pressure, drum-on-tire pressure, same tire), then rear wheel speed can be a decent proxy for power.
If you want to use your Polar for speed on a trainer just mount the speed sensor high on a seat stay and your Polar watch upside down hanging from the front of your saddle (use paper clips or be creative). Put your head down like you’re looking at your back brake to get a reading.