I use red. The green is marginally brighter, but for me the red just seems to contrast better against a wider variety of targets - including when it is not at optimal brightness. In those lower light conditions, the green seemed yellowy and washed out - more like a hole in my front sight, than a part of it.
I'm willing to concede there may be some confirmation bias as red/orange colored front sights are simply what I'm used to seeing. I think either is plenty bright.