Arctic White without a doubt is the "whitest" of the two.
I have a Diamond White Metallic SLK built and awaiting a ship for delivery. I spent a lot of time looking at the two colors in person on a dealer's lot in different lighting conditions. Online pictures will not be able to show you what is going on. Color chips in brochures look nearly the same (Diamond has a bit more gray perhaps), and most MBZ iOS apps I've stared at for hours don't even attempt to change the color between the two.
In real life, Diamond White has some gold, perhaps silver, metallic flakes that sparkle in the sun, yet looks almost matte -- an "off white" or "cream" (the color of milk, NOT tan) if you will -- in the shade. If you're worried about too much bling, the sparkle of the MBZ Diamond White is not as great as what most Lexus have in their standard metallic finishes, certainly not as much as I loved in one of my past pearl white rides. I did not see any noticable increase in orange peel
with the Diamond White compared to Arctic White (unlike what I had on my previous BMW). Beyond the premium price uplift of Diamond White (at least on the SLK here in the US), when I read about this color across the net and people in multiple countries chiming in, the "off white" look is what some people didn't like compared to the traditional Arctic White. IMO, Arctic White is just "plain white" no matter what sort of light you view it in -- it's also a base color with no price uplift if that is a consideration.
According to DuPont
, 24% of all NA cars were white last year -- the most popular color once again, so I'm excited to soon have Diamond White Metallic and be just a little different, with a bit of bling compared to most on the road. My suggestion? You need to see both colors in person. No online pictures or descriptions are going to do this subject justice.
Good luck with your GL decision!