There are several pull up bars, all of them are very effective. There are generally two types of pull up bars with some exception. There are the pull up bars that have to have something mounted to the wall to help support it or, there is the kind of pull up bar that when you do pull ups that your body weight pulls it down on the door jam. The exception to this general rule is the Universal Door Mount Pull up bar by : EasyEffort.
This particular pull up bar has six different grip spots so you can do up to six different pull ups. This bar gives you the choice of six different pull up work outs. You can do pull ups with the narrow grip, pull ups with a wide grip, pull ups with the neutral grip with your palms facing, pull ups with a medium grip, pull ups with the shoulder grip and pull ups with a extra wide grip. This pull up bar hangs within and over your door but has extra foam over the pieces to keep your door from getting messed up, which is a big plus because a lot of these bars mess up your door jams.
Then there is the free standing pull up bar. This bar not only makes pull ups easier to do but if gives you a chance to do other exercises while doing pull ups for even more of a work out. This free standing pull up bar doesn't take up a lot of room and you do not have to put any holes in the wall and you do not have to worry about this machine messing up your door way. I also think that this machine over all helps to give you more of a choice when it comes to your work out without having to worry about permanently doing any kind of maintenance to your house. I would have to over all say that I would prefer this machine just for the fact of the options workout.