If we're thinking about the same "trap" bar I think Rob NC's on the right track. With the trap bar the bend in the bar allows more clearance for your knees so you don't have to bend as much at the waist, that allows you to use more leg muscles to do the lift. With a straight bar you have to bend more at the waist and that works your back harder. Like tjwes said, try both and see which one targets the muscles you want to work.