Get the metabones one. Its half the price of the novoflex and its actually nicer.
The system it uses to change the aperture is smoother and more accurate (longer throw) than the novoflex. Even if you have lenses with aperture rings, it allows you to change exposure mid shot since it is unclicked and dampened.
It also has a removable tripod foot which is good for taking stress off the camera's lens mount with heavy lenses.
Look into a speed booster too. Definitely a worthwhile investment if you have a whole set of F mount glass. Basically turns all your lenses into two lenses.