I jump seated on a 737 with one of the airlines highest seniority pilots. He moved from 767s to the 737 as it took him from being one of many high seniority pilots to being the 3rd highest seniority pilot in the fleet type. He basically is guaranteed to get the schedule he wants (with in what's offered) where as on the 767 he estimated it was 70/30 in getting what he wants.

A college classmate was a very low seniority FO when she got a spot on the 787 when it was introduced do to it having a very small route network which made the scheduling very rigid so lots of people didn't want it.