Why are you out for a ride?

Is it to look at the scenery or are you trying to improve your fitness level in the most efficient manner possible?

If you are just stopping to smell the roses, you don't need one. If you actually care about improving your fitness level, you would do better to have some quantitative, objective way to measure your progress. That said, if you had to chose between the computer and a HRM, the HRM is far more appropriate for gauging fitness level and exertion.