2. The close-up lens magnifies the image before it enters the main lens. This brightens the image and there is no loss of light. Extension tubes cause a loss of a stop or so of light, as the effective aperture gets smaller when the distance between the lens rear and sensor increases. Considering quite a lot of macro photography happens in low light conditions, the loss of a stop of light may not be that trivial.
3. Extension tubes with electrical contacts help retain auto-metering and aperture control of the camera. So, if you decide on using extension tubes, stay away from the cheaper ones without electrical contacts.
4. The longer the focal length of your camera lens, the longer the extension tubes you'll need. So you may have to keep adding tubes depending on the focal length you choose to shoot at. Also, while using with a zoom lens, one may lose focus while zooming, which may be annoying. Close-up lenses are far easier to snap-on and it’s a one-time process irrespective of focal length of the lens.
5. Extension tubes, though, are independent of type of lens and can be paired with pretty much every lens. But a close-up lens attaches through threads and will have to be matched to lenses with the same thread size. An option is to use step-up or step-down rings to get to the correct thread size.
6. Good quality close-up lenses are not cheap. It’s far more economical to buy a set of good extension tubes.