I would add Sigmasport to the mix just to confuse matters! Very good computers too.

Get one with current speed, max speed, average speed, trip distance and trip time (most good ones will have all these anyway). Cadence is useful if you're inexperienced and can stop you labouring in the wrong gear for example. A good cadence to have is generally between 70-90 rpm. If you want to get really technical you could get one with a heart rate monitor.

Enjoy!