Much less is known about them, but I believe that the testicular one is a harder silicone. I don't believe it's gel, but if it were, the amount of gel would be much less.
One of the problems with breast implants is that there is so much silicone. If it breaks and leaks, there is an enormous amount of silicone that can get into the body, whereas if you have a very small implant, like a testicular one, even if it were gel and did break, the amount of silicone the body would be exposed to is much less. Therefore the chances for problems would be fewer.
Also, the number of men who get testicular implants is very small compared to the number of women who get breast implants, so we know much less about them. When you have millions of women getting a product in the United States, you see a lot more problems. Even if the proportion is the same, it's easier to keep track.
Also, for women with breast implants, it's taken 40 or 30 years to start talking openly about it. I don't think men with testicular implants are speaking very openly about it yet.