An extension tube doesn't have any optics in it, it's just a tube that is used to move the lens in relation to the sensor, because there's nothing but air in there all the extension tubes in the world will use the same air will give you the exact same image.

There are 3 ways extension tubes differ:

1. Electrical contacts

   Tubes with no electrical contacts (the cheap kind) don't have a connection between the lens and camera, you don't have auto focus and don't have aperture control, the camera will only work in M and Av mode.

   To control aperture switch camera to Av mode, set aperture, press DOF preview button and hold, while pressing detach lens

   Tube with electrical contacts: those are easier to use because you have all the normal camera controls (but I don't think auto-focus will be useful here).

2. Thickness

   Thicker tubes will increase magnification and move the focus plane closer - but remember if the tube is too thick the focus plane will move behind the lens's front element and nothing will be in focus.

   You can buy a set of tubes with different thicknesses that you can combine to have a large number of options.

3. Build quality

   Just don't put an heavy expensive telephoto on a thin plastic Chinese tube.