I think the 650 seat configuration will only be used to places like BOM, CMB, KHI, possibly DAC and MNL; EK will have three separate configs - one for JFK, which I think is about 479 seats and another for LON, about 500, so there will be an opportunity to try out a number of different routes.
And while I can't say anything about EK's internal workings or how seniority works, I would only echo what Super Duper says; it would be a privilege.
It kind of reminds me of what things must have been like in the late '60s and early '70s in airlines about to receive 747s (also late, in most cases!) - do senior FOs go for the 747 or their first commands? At least an A330 is a little more attractive than a 727 or a 737, but still, a chance to fly the biggest of them all ... that's something to tell your grand-kids about!