Is that really the question? Well, the answer is both - it just depends on the situation. Last week I participated in a workshop given by Neil van Niekerk, flash guru. The morning was spent going over some flash principles and the afternoon was spent photographing 2 models in a few different locations around San Francisco. Our assignment was to mix flash with ambient light to keep the subject from looking like flash had been used. Natural, available light is wonderful but it isn't always enough. And if you don't know how to use flash properly then you get hard shadows and a very flashy, not very pretty look. Neil has mastered the art of the flash and his techniques are actually pretty simple. I have read everything on his blog about flash and it helped a lot but this workshop made a world of difference. I also met some great people and got a few shots that I think turned out pretty well. The first shots were indoors and lit by bouncing the flash.
The next shots were taken outside in Maiden Lane using a flash in a softbox triggered by Pocket Wizards.
Of course having lovely models doesn't hurt ;-)