Cadence monitoring or heart rate?

Rulebritania Posts: 209
edited August 2013 in Road beginners
I have a garmin 800 and whilst I am limited to the amount of time I can spend on a ride (medical). I would like to make the most of the time as it stands I average 17mile rides and occasionally upto 30miles. Which should I get if any to get more out of what I can do. :?:
Don't call me sir I work for a living