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.