Actually the weapons are about even, astromech and system upgrade are roughly equal, Torp options are the same, Turret and cannon upgrades are relatively comparable. The dial and stat line are better for the B-wing but that is balanced by the, what, 6 point difference between their lowest skill pilots.
The baseline 3 Attack vs. 2 is a very big advantage for the B-wing.
I think Advanced Sensors are better than the Astromech slot, too. The R2 Astromech is great on a Y-wing, but mainly because it compensates for the horrible dial. Most of the other astromechs are decidedly meh. R2-D2 is limited by the lack of green, the generic R5 may see more use with more crit-causing options in play these days, and both named R5s continue to be pretty bad. Stacking those up against Advanced Sensors and Sensor Jammer doesn't end well for the Y-wing.