High or low GN for fill flash?

[TiCoyote]

I generally don't use flash outdoors, but I have an outdoor portrait session coming up, and I'd like to use some fill flash and maybe a bit of overpowering the sun.

I have a Canon 600 and a 430. Do I need a lot of light to fill? What about to overpower the sun? Which flash would be a better choice? I know that the 600 will put out more light, but the 400 is smaller and lighter, so it would be an easier option.

 

[Soocom1]

Counter acting the effects of the sun requires closeness.
Watch the aperture settings and be ready to turn on/off the flash quickly to gain the shot.
I had to use a flash in broad daylight for a wedding and it worked well so long as the effect is close.

 

[Derrel]

Your flash Power should be about 2.5 to 2.7 stops below ambient light exposure when you are using the flash as fill Flash. You do not want the flash to overpower the light that is ambient on the scene, otherwise you are no longer shooting fill Flash, but have transitioned into using "flash as key light".

 

[Derrel]

I would make my decision on something other than strictly the guide number of The Flash. A bigger, more-powerful Flash will usually recycle faster than a smaller Flash, especially when it is turned it down to 1/4 or 1/8 power, so you might find that the Canon 600 flash is actually the better tool to use, despite being larger and heavier.

 

[JBPhotog]

I like 30% flash contribution however 50% can look quite natural. I of course use a light meter to determine this.

See this for examples, Fill-Flash Tutorial