One is vandalism, which is a crime, and one is not.

There, we have at least one delta now.

If it were up to me, billboards WOULD be a crime - and in some places that take pride in their land's beauty, they are!

Another difference is that one of them is intentionally designed to capture my attention, usually while I'm driving, to try and get me to fork over my money. So that's another delta.

I don't love graffiti, but I do hate outdoor advertisements.

I much prefer graffiti to advertising. Sure, I wouldn't mind if amateur tags disappeared, but more intricate tags and mural-like designs that prop up from more experienced graffiti artists are something I enjoy, and they are part of what give the city it's vibe.

You must live in Paris.

That's why I said "almost". But when discussing whether one should be the crime (or whether the other shouldn't), it's hard to go beyond one makes people decide to buy something and the other doesn't. Which of course also invites its own discussion about its value.