its good to use BOTH.
Straight bar your hand is rotated out contracting the 2 heads a lot more. At the top of the curl you are forsed to squeeze the heads to finish the rep.
Good for over all power curls and looking impressive in the gym.
EZ bar is easier on the elbows, forarms, wrists and grip. Thus the name "EZ Bar"
EZ bars have a wide variety of styles and bends this also allows more variety of grips and angle to work the upper arm and hit the Brachialis and Pronator Teres also giving more size and thickness to the arm.
Boils down to goal and length of time under your belt. With this being in "Muscle Building and Bodybuilding" I'm assuming you care about the end result look of your time invested.
If you can use both, alternating them. Do EZ for month then us a straight bar for a month. With either one you are still doing bar curls, just keeping it fresh and getting the benefits of different angle every other month.