My bet is on the A350-900.
The above shwon LH chart is not quite correct as it just shows the seats of the used seat layouts. You could squeze in over 300 pax into an A330/340 easily, but at the expense of First and Business class. All economy even 440 is possible. (same as A359).
I think Swiss could use the space of an A359, especially since they will be having First and a big Business class.
Furthermore out of a passenger point of view I prefer the A350 over the B787.