I agree. I try to buy the longest set I can afford. Idealy, a 25ft. set would allow you to jump a vehicle that may be stopped in front or behind you--say on a dangerous stretch of highway.
The lower the gauge number the better. Pick a set that have both lead wires joined, so you don't have to untangle 2 wires everytime you take them out of the trunk.
Also, if you live in a region where you get cold winters, make sure the jumper cables are rated for sub-zero temperatures. In really cold weather, the cheaper cables will cease being flexible. The better cables still stay malleable within their given rating.
- Frank