I'm going to say LB. I know it's not magnetic, but I had a Sterling Gray Metallic on my '14 and I feel safe in saying that dirt shows more on LB than on a darker gray.
i have magnetic and i dont think dirt shows that bad at all. i did have a deep impact blue GT350, and it seems it did show dirt more than the magnetic. That being said. when i order an 18, im still debating on color i want, and its between magnetic, lightning blue, kona blue and Ruby red right now, more leaning towards lightning blue right now.
Magnetic is closer in color to dust and dirt than LB so it will show less. You will think Magnetic doesn't look that dirty till you wash it or someone parks a clean Magnetic next to it.
I know there was a thread about the Magnetic vs LB and it was discussed about which shows the dirt more and I did reply in that one that LB did for sure since I had a '14 SGM stang. The pic above of my car is without washing for two weeks. It has rain on it, dirt, and pollen. If you zoom in, you will see spots and what not. But after posting the pic and seeing how it looks from even a short distance, I must say it still looks damn killer to me! @bjstang thank you Bill and that is my name as well