One very important thing left out of cachehiker's reply - it depends on the 'type' of cantilever brake...

If you have a 'low profile' (like modern shimano, Avid etc.) brake then lower is more power/softer feel and higher is less power/firmer feel. How low? Real low. THe lower the better. If you don't have fenders then get the cable so low that the fuzzy molding flash on a new tire will rub on the cable yoke. Then you will see how much power is possible in a rim brake.

If you have a 'high profile' cantilever (like old Mafac or Paul's Neo Retro or SRP) brake then lowering the straddle cable will not give you more power - you want the angle between the straddle cable and the arm of the cantilever to be close to 90 deg. when the brake is engaged, so these ones pretty much need to be high. I don't think these would work well at all with a fork mounted cable hanger.

edit: www.sheldonbrown.com has two excellent articles on cantilever brakes.

Here is the theory

and

here is the practical