Agreed, black is a bit of a pain to keep clean but if you are an enthusiast you will always look after your GTI, regardless of colour. IMHO, metallic is the way to go as flat colours, especially red, will oxidise in time and turn a hazy pink - the wife's Polo did and once you've cut the paint, it never seemed the same again. You need to ensure it has good wax protection to prevent this happening.
With darker colours, you need to be very careful in the washing process as small scratches and swirl marks show up more in sunlight. Never, ever use a car wash

and make sure you use the best polish/wax you can afford. When it's polished and waxed, black is the dogs

Cass