Arguments for both have been made but the main question is what do you consider to be justification for a language. Look at
www.ethnologue.com,
www.ancientscripts.com,
www.omniglot.com. The generaly accepted definitions of languages change. If you have a good english dictionary that covers English english and Amerian english look and see that some word have different meaning, pronunciations, spellings, and some even are absent. these pages show many of the differeces
http://www.scit.wlv.ac.uk/~jphb/american.html
http://www.accomodata.co.uk/amlish.htm
http://www.edupass.com/english/dictionary.phtml