I have a Garmin Forerunner 301 XT watch which gives me speed information. I wanted to correct my cadence so I was thinking of buying a cadence monitor. But I don't see the point of buying that extra speed sensor. Am I missing something here ?

Another question:
I have two options for a cadence monitor:

- Garmin Bike cadence sensor
- older Garmin GSC 10 speed sensor

From what I found out, the first one is all self contained and has no external parts. But, I was wondering if anyone here has used either of these and would recommend getting one over the other ?