I have been looking for a Benelli M1 super 90 or M2 tactical and as far as I can tell there really is no difference is there?  I know some of the super 90s were HK imports and had factory tube extensions.  I can't find a new M2 tactical w/ pistol grip anywhere and no one even wants to order one, but I know where I can get a used super 90 that is in good shape for a desent price.  

Also, what would you prefer for sights?  Rifle or ghost ring?

For the most part, no.  No real difference. M2 is just the current version, M1 is no longer made.