Personal opinion (and worth only slight less than you paid for it):
Gray tints are usually recommended for flying as they don't distort color. I think that's a good suggestion if you're flying at higher altitudes. Up there the air is clear and the sun is bright and you're usually not navigating by pilotage.
Most of us private pilots, though, are flying below 10,000' much of the time, often quite lower. At lower altitudes the air isn't as clear; there will be haze, smoke, etc. Also, we're often flying VFR and using pilotage, so we have to look down through whatever obscurants are in the air and see the ground clearly. Amber tints filter out some of the blue light and provide better vision and contrast through haze.
So, in my humble and oft ignored opinion, you should choose the tint based on the type of flying you do. If you'll be low most of the time, go with amber. For me, an amber tint works very well.
On a related note, also think about the gradient you want. I had my lenses custom tinted by my optician. My gradient is pretty drastic, being quite dark in the upper third and almost clear in the bottom third. I really don't want tint when I'm looking down at my panel or trying to read a display on an iPad or phone.