They're very similar, Tara but the tomato hornworm has a dark brown "horn" when they're mature, and slightly different markings on its body, more like spots than stripes. The ones with the red "horn" and diagonal stripes are tobacco hornworms.
I had to look it up so that I could tell the kids at school the right info. Even Wikipedia's entry for it has the two mixed up, and pictures of both types in there.
