If said cake is german chocolate with maximum fluffiness and biscuit is a cookie, it depends. If the cookie is a cookie monster cookie, I'd choose the cookie. If its anything else, the german chocolate cake. :9
Astra- did you know that Jaffa cakes used that fact to argue that Jaffa cakes are indeed cakes not biscuits, because different VAT rules apply?
Now for the question in hand, if I have a cup of coffee it has to be a biscuit cos I like to dip. Also I'm a bit scared of cakes at the moment cos there was some yummy fudgey gooey chocolatey ones in the staff room 2 days ago, and my BMs are only just getting back to normal....