Between Romeo and Juliet and West Side Story... hmmm....
I know West Side Story is presently based as a post-modern version of Romeo and Juliet. The Montegue and Capulet clans are supposed to be like the warring gangs in West Side Story-- I think they were called The Sharks and The Jets; it's been a long while since I've read West Side Story, so I can only be partially certain of the names of the gangs.
Superficially, the do resemble each other... the same way Romeo and Juliet fell for each other, so too, did the two warring rivals in West Side Story (again, it's been a long time since I read West Side Story, so I can't remember the kids' names). And other than the two killing themselves at the end of West Side Story, I can't see any real distinctive similarities between the two stories...
They both share a sense of betrayal of their clans/gangs, for the tryst between Romeo and Juliet and their dopplegangers for West Side Story; they both have their sense of anger through the clans/gangs for said betrayal; both clans/gangs had losses on their sides, and both of the main character's killed themselves at the end of the play/movie. Other than the fact that one was a play, and the other was a musical (which I think is one of the lowest forms of entertainment-- The Rocky Horror Picture Show, being the only exception to the rule in my eyes, as it made my laugh my butt off), I can make no distinction of how anyone could ever think that West Side Story is even on the same level as Romeo and Juliet; William Shakespear must've rolled over in his grave, when that film was made.