It is true that simply deleting an email or text doesn't immediately overwrite the data but the space that was used will no longer be marked as in use and at some point will get overwritten.  Even if it has not been overwritten, finding it may be quite difficult and require resources that LE is unlikely to spend unless there are more serious crimes involved.  Certainly, you should not keep emails and texts around but I don't agre that they are as risky as you think.  But if you want to be safer, then voice is the way to go.