For simplicity and cost, if you don't need something that displays maps or tracks, I'd go with a unit that reads wheel revolutions. Swap out the battery (or batteries, usually something like a CR2032) every year, logging your mileage before replacing the battery. You can use the unit's instructions to set wheel circumference or ride it over a known distance and make adjustments to match. If the circumference is accurate, the other stats are likely to be accurate, since they result from math applied to the # of revolutions and the elapsed time.