I voted IAP Light as this is what I had been using since the new options were available. However, I am now using IAP Grey as recent vision problems spurred me to check which options was better.For me, the IAP Grey seems better.
I never new this option was even there. I use the default, because its the default. However, I checked the others out and think they would be great in dark or low light areas that you don’t want to disturb anyone else. Such as on a long international flight, which I will be taking soon.
I think for my morning reads, I will be sticking to the default/light setting. I find it brighter and easier on the eyes.
I prefer the Gray version. There's enough contrast to make it easy to read and dark enough not to disturb my wife if I'm on my Kindle late at night.
I just noticed that we don't need to say what choice we've made as just clicking on an option shows who else has chosen that one too.
What I also like is that it is so easy and quick to change. No having to go into your profile and mess about in there.