I've looked around the internet for a side by side comparison of these two strange pieces of equipment and haven't found anything very helpful. Hopefully this will help some of you who may be looking at macro alternatives.
About a year ago, I was looking at stepping up my "ring shot" game as I primarily shoot weddings and the ring is definitely a detail that should be captured well. It seems like most wedding photographers turn to the Canon EF 100mm f/2.8L Macro IS USM. I thought that seemed pretty ridiculous for one shot a day. Here are my thoughts on the 100mm Macro... A. I don't want to carry around another L lens in my bag, the less gear I have to carry the better. B. I don't want to spend almost $1,000 on another lens that I'm only going to use for a handful of shots each wedding.
I see how people are drawn to the 100mm though. It can be used as a portrait lens in place of the 85 or 135 if necessary. It's a true macro lens with the ability to focus to infinity. These tubes on the other hand are limited in their capacity to focus. They have a minimum and maximum focus distance which varies depending on the lens they are used with.
I first picked up the EF12 II (the smaller one on the right in the image above) used off B&H. Since there is no glass in either of these Extension Tubes I didn't mind buying used in the slightest. I used it for a while until a friend bought the EF25 II (the larger one on the left in the image above) in which I saw more potential. It took me a few months to pull the trigger on the EF25 II, but eventually I got it and was stunned at the difference. The EF25 II seems to be better suited to isolating rings than the EF12 II. I think both have their place. The EF12 II allows me to pull in environmental details while still having the rings be a decent size in frame without cropping. Below are some sample ring shots in order to help compare the two. For these shots I used the Canon 50mm f/1.2L with either the EF12 II or the EF25 II attached.
None of the following images were cropped or sharpened. All images were shot at minimum focus distance in order to provide consistency for comparison.
In order to provide a little variety, I changed the background up and shot these next images from above.