Scientifically speaking, amber is "easier" for your eyes to see than red. Amber "appears" to be brighter at the same intensity as red. Think of a red digital bedside clock vs. a green one. A green digital clock will light up the whole bedroom, while a red one doesn't cast any visible light.
There MAY be an issue with washout in certain situations though, which is why a lot of guys choose red. I had a TR21 in red, and it was GREAT in the daytime, but I did notice that at night, it was difficult to see at first. If I get another, it will most likely be amber because it would go on a rifle that gets used for shooting armadillos, not "SHTF". In the daytime, if I can't see the amber reticle, I'll use the black pointer.
It is personal preference since our eyes do see color differently from one person to the next.