I used to agree, until my elbow tendons got painful. With the EZ bar, I can add 20-30 pounds over the straight bar and curl it with no pain. So for me, the EZ means "more mass".
With the straight bar as with everything its how you use it that matter
Take it from a "biceps" man
The why people some times have issues with the straight bar is the hyper extend at the bottom of the movement and the curl in a static manner, holding the bar arms stretched at the bottom of the movement for 1 - 2 seconds (kiss of death)
The ez bar is ok what I fing is it neglects the in we head of the bicep and puts the most stress on the outer head, which is what I use hammer curls for, so it does t give that thick look overall