The straight bar will put your biceps in a slightly more supinated position, and therefore stress your biceps slightly more than a cambered bar (z bar). The cambered bar pronates the wrist slightly, therefore engaging the brachialis and brachioradilais muscles slightly more.
The main reason people use a cambered bar is that it takes stress off the wrists. If you have wrist pain when using a straight bar, switch to a cambered bar, and stretch the wrist flexors to gain additional range of motion.
SO, the straight bar is slightly better for the biceps muscles, but not much. Use whichever on you find more comfortable, IMO.
