For most applications, timing chains are the best route. A double roller timing chain is going to be the cheapest and will last a very long time in most cases.
Gear drives provide the absolute rock solid timing. There is nothing to stretch or wear out, short of breaking some teeth. However, like said, there is a gear whine. Also, it is possible that the vibrations emitted by the gears can mess with the knock sensor. I've never used one personally, so I don't know.
Belt drive, the best of the best. The pros are they are for the die hard racers. They emit the least about of noise or vibrations. The downsides: breakage and cost. Also, keep in mind, often times a special timing cover is needed (wet belt drive). So road matter may cause damage to a dry belt system.
My advice: double roller timing chain, like a Cloyes, Comp, something.