I've had both, currently with Phantom Black, and I would say that I think that i prefer the Brilliant Black. The Phantom looks very nice, but I think that I want the metallic flake to be more dramatic. You only really notice the metallic look if you are right next to the car, with the sun shining on it and clean. Even when spotless and in sunlight, the parts of the car that do not catch direct sunlight look black. You don't really get that deep, deep sheen that you might see on a silver.
The Brilliant Black is so nice and deep when clean, though. And it is a little bit more mirror-like than the Phantom, although both do a good job of that.
However, both are equally as brutal to keep clean. Maybe the Phantom hides ambient dust a day longer. But you have to really like keeping your car clean for either of these blacks. I try to hand-wash my car once a week, so I can say my car looks clean more than 50 times a year! 8-)
To the scratches: even dust \ sand @ highway speeds leave mini swirls in the paint.
I have phantom, I like the deep colour look vs the really shiny of the brilliant. It's only really obvious in sunlight... however if you are next to a black car you can really notice the phantom. I'm not going to go as far to say it looks gray, i'll say it's not a "pure" black. Might have an ever so slight grayish tint when the sun hits it, but again just enough to say it's not pure black.
I had the same issue while ordering. I looked at Meteor Grey (too green\blue in light), Deep Sea (too "grandpa\old" blue), Brilliant (just black), Phantom (cool black effect I was looking for, but had a little "imperfect not quite black" look and Lava Grey (feel in love with this paint, custom ordered, I would of gotten this but it was 3500 more, not worth it at this time... maybe on my future RS6\4 :P)
Phantom Black, in my opinion is not "grayish" but more of a "not pure black". It's black at night\dusk\dawn. IT does not look "dirty" in the sun. Has LOADS of blue\purple and green flecks. All an all i'm happy with my choice. However I started out that I didn't want "just black".