i think bent arm dumbbell pullover is a better exercise than straight arm dumbbell pullover,but if you like to do the straight arm pullover,then,here is a way of improving on that exercise.do your set of sraight arm pullover,when you reach your last rep,immedietly and with the same weight start doing bent arm pullover,you will be able to get a few more reps,you will benefit more from doing the pullover this way.