One of the things John doesn't mention is why most people take the interior route south. Road condition! Unless things have changed in recent years, the 80, 180 route down the east coast is a real bummer! It was by far the worst road I've ever been on! We had to get past the city of Veracruz in Veracruz State before it got better. I swore if I survived the trip I'd never take the coast route again; and I haven't. So John probably has more current info on that.
And look out for the cops around Tampico. Almost all of them are looking to shake you down for some mordida (Spanish for 'bite', but in this context it means bribes). See my website for a complete run-down of our east coast trip (or at least most of it, I'm still working on the Caribbean end of it).