It's all a preference thing.
Some advantages of different modes:
Av : Will automatically set shutter speed for you. In flash photography shutter speed controls ambient light. Most of the time Av will give you a good balance of flash and ambient.
Manual camera controls: Well ofcourse you know that it gives you full control, helpful especially in situations that might trick the camera, such as extreme backlighting from the ambient source.
iTTL flash control will give you the amount of light necessary to illuminate your subject. Most of the time it will be pretty accurate. You can use FEC (flash exposure compensation) to adjust the exposure on the subject, giving you some control of flash/ambient ratios.
Manual flash control: once again, full control, always a plus. However, this comes with a drawback. You'll most certainly need a handheld flash/light meter to meter the light, although you can do some math based on the flashes guide numbers to calculate exposure yourself.
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Personally, I shoot full manual, I don't even own a TTL flash. I use a Sekonic L358 light meter to meter flash exposure and usually use the camera's built-in meter to meter the background. This gives me the most control, which is good for the kind of work I do.
However, my method is not the greatest for subjects I don't have control of, because I have to do constant adjusting. If I were doing journalism work, I'd more likely shoot iTTL in aperture priority.
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