YABT-Which is better to monitor?

[JulesMaximus]

I want to get a heart rate monitor for cycling but I was talking to the guy at my LBS and he suggested a cadence monitor instead or one that does both (which are more expensive of course). Anyway, I thought I'd ask some of the bike gurus here. I'm thinking heart rate because I'd like to see where I am as far as my target heart rate to see how much I'm pushing myself. I'm not racing, just fitness training. Miles ridden, ave mph and heart rate are important to me.

Am I overlooking something here?