Tiking,
I kind of get where you're coming from - Brilliant Black looks great on a new car, and from a distance there's probably little in it between that and the Phantom Black.
I debated for some time whether I would be happy with the Brilliant Black (and save myself £600 to boot!), but my mind was made up when I saw an A4 in Brilliant Black next to a Phantom Black one on a nice sunny day. They were both on the dealer's forecourt as second hand, but were obviously nice and shiny. The Phantom Black was much nicer and the metallic flakes in it really set it apart from the solid colour.
I also suspect it might wear better as the metallic is obviously a 2-layer paint with a clear lacquer over the metallic part, whereas the solid paint has the colour in the top layer of the paint. Metallic is more expensive if you need to touch up or repair/replace any panels though...
I'm also getting the Autoglym paint protection treatment done as I had a similar product (A-Glaze) on my old S4 and I never had to polish it - the water beaded on the surface after just a quick wash, and I often left bird droppings on the paint for some time with no ill-effect to the surface of the paint. All this after 53k miles and 4+1/2 years and the car came up like new after a wash!