There's no single answer to this other than "do what works for you". While the optimal answer to this question is to have both a helmet light and one mounted on the bike, not all cyclists have the money to do that.
That said, there are a few things that can help you decide, Mac or PC helmet- or handlebar-mounted light:
Will you be using more than one bike? In this case, the answer's clearly "yes". If you bring the light with you, you can get one great light setup as opposed to several less-expensive lights. (You can do this with rear blinkies as well as headlights, but a blinky on your helmet is best as one that's supplementing a properly aimed blinky mounted on the bike.)
Do you commonly need to direct light in places other than right in front of you? If you ride on a lot of twisty roads or paths, often need to read street signs, need to watch closely for potholes, etc., then a helmet-mounted headlight would be a handy thing. If you usually ride roads you know well in good repair, you may find a helmet light less useful than one mounted on the bike.
Is the weight on your head a problem? This is something you can really only decide by trying it out, or by attaching the equivalent weight to your helmet and riding for a bit. Are you prone to neck problems? Does the weight make your helmet chafe? (It does with mine.) This problem will be more noticeable if you often ride offroad or on bumpy roads.
In the end, you'll have to try this both ways and see what works for you. You can give a helmet mount a test-ride by attaching the light to your helmet with a nest of zip ties, duct tape, and velcro. If you like having the light on your head, you can look for a more elegant solution.