Answer:

The main difference between the EZ and the straight bar for curls is that with the EZ bar your hands are slightly pronated (thumbs are higher than pinkies), changing the pattern of activation of the upper arm muscles.

Essentially, what this means is that your biceps are getting a little less work and your brachialis muscle (which lies under the lower portion of the biceps) is getting slightly more work because of the angle of your hands. This makes the straight bar somewhat more effective for building your biceps. You can definitely still get a great biceps workout with an EZ bar though.

One very good thing about the EZ curl bar is that it is easier on the wrists than a straight bar. In fact, the EZ curl bar was actually designed for this purpose. It is good to mix the exercises up in your biceps routine so keep the EZ curls in.

The most effective position for your biceps is the straight bar with a narrow grip. This means gripping the bar on the rougher surface exactly where the center smooth part ends (should be just inside shoulder-width).

To really maximize your barbell curls, try this pull-in technique.