My (admittedly limited) understanding is that corporate personhood conflates individual, corporate and commercial speech. Attempting to treat them as separate things might work philosophically, but seems like a legal dead-end. (If you were attempting to speak from a philosophic rather than legal perspective, ignore this paragraph.)
I don't see why a corporation should have any less right to free speech than an individual: governments can step on the toes of a company just as easily as a person. Nor do I see why commercial speech should not be protected: I could argue that nearly everything is tied to making money. (Painting a billboard may not be explicitly commercial, but the more exposure people have to such art, the more likely they are to buy it. It's another form of advertising, even if that's not the artist's intent.)
Property rights trump speech rights. If you are on my property, I can tell you to shut up and go away. If I own a wall, you do not have the right to draw on it.
Nobody is forcing anyone to look at a public billboard. It may be hard not to - but it's also hard not to listen to people preaching on street corners. These preachers are probably just as emotionally manipulative. I wish they weren't there, but I'm glad they have the choice.
Edit: I don't think I was very clear. My main point is that if you draw a line between what speech should and shouldn't be protected, I think you will always end up with some speech which needs protecting but is on the wrong side of that line. It will be the kind of speech which a lot of people don't like, but I think that's the most important kind of speech to protect.