"Illegal" is merely descriptive. "Wetback" is pejorative.
I think "illegal" does at times have that negative connotation as well like the "w" word; but this is really a linguistic debate. Both arguments can be seen as correct. And "illegal" is descriptive as well, it has caused some flack though in use. Some people prefer "undocumented workers" or things like that. As I said, it is a linguistic argument and it might be different in Texas than in California who, the latter, I think are being loony to a terrible extreme.