Well I can't say anything definitely, I just got a book the making of West Side Story so I think that may give me more info...
I know Bernstein wrote One Hand One Heart
I know Sondheim wrote I Feel Pretty
Something's Coming came fairly late so my guess is it was either Sondheim or a combination of Sondheim and Bernstein
There might have been quite a few that were a combination, but one would be given the credit more so, and b/c Bernstein took his name out of lyrics so Sondheim could get his proper credit, my guess is Sondheim.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli