Both extension tubes come without any form of cap or protection case; something that I did expect for an otherwise high quality item.
Practically speaking the working distance is the most useful as it shows the minimum distance in mm between the lens barrel and the subject. Browse the table to see if either the 11mm or 16mm version will best work for the specific lenses you want to use it on. I personally would not plan on using a combination of lens and tube if the working distance is less than about 20mm. Otherwise the lens element often creates a heavy shadow on the subject or might even completely block the light.
Some of you might find the price tag for the MCEX-11 and -16 on the high side for a something that just "spaces" your lens further away from the camera. Given the high quality the rings, I do recommend getting at least the MCEX-11; for less than 100,USD it might even boost your creativity.
If you are not sure, the MCEX-11 mounted on the 27mm pancake lens is a great combination to start with. Because the lens is so small the working distance remains pretty high at 76mm for the -11 (60mm for the -16). Otherwise I did also like both the -11 and -16 version on my 18-55 kit-lens as long as it is not used at the wider focal ranges.
To me the tubes are not a real replacement for real Macro photography but just another tool in your camera-bag. Their small size and compactness can come in handy when the unexpected need for a closer focus distance and higher magnification is called for. A good example would be a wedding photographer who takes a few shots of the wedding rings with an extension tube, rather than using a dedicated Macro lens.