Unless your interest is only in Scandinavian languages and culture, my two cents is that you learn modern German first as it is the language of such a wealth of texts.  Icelandic can be a stepping stone to the Old Norse languages, but unless you are interested in the modern Icelandic novel or the daily news from Reyjkavik (which you very well may be) there isn't going to be a whole lot for you to read.  I think that availability of personally-relevant texts is an important criteria for the language-learning student, and I do not mean to offend any natives or fans of Iceland.  On the other hand, if you really like the films of Ingmar Bergman, then perhaps Swedish could be a relevant modern tongue for you...u.s.w...

 

-Ian

(my first post)