It's better not to compare the two. Think of texting as a technical language, which has become associated with the other traits of being a teenager.
Consider this: does the technical language of accounting, engineering, science, politics or religion hinder a professional's command of English? You might answer "Yes" if you've met a professional who used too much jargon in their writing.
Similarly, if texting is used out of context, e.g. in a formal essay, you might also think that the teenager's command of English has been affected.
I would argue, however, that teenagers are quite capable of learning both texting and formal English as well as learning when each is appropriate. They just need training and practice.