I think the design is worth more, because that is what the audience sees, anyone can code a half ass design that functions decently, but the design is what brings in the clients.
I would probably say the design is worth more, although it does take both to make a vbulletin skin, you will be able to find 100 coders to every decent designer.
People don't think about the coding when they look at a forum, they see the design and go "wow, great skin, who designed this?".
Imo, it's designing.
But I guess it's just because I cant code :P
And I dont know how hard the coding part is, but I know designing is.
Also, designing is much more "flexible", when coding, you already have a "plan"
Just my 2 cents