I tried the SizeGenetics extender, and was very disappointed. Very uncomfortable, very expensive.
Personally, I think extenders are very similar, with only small differences. Don’t buy the cheapest one on Ebay; it might be shipped from China (making you wait for months), or have a defect, etc.
If you like it, you might want to add a VacExtender add-on kit from Monkeybar. He sells a simple vacuum attachment for Jes extenders (and copies of them) that is much, much more comfortable than the traditional extender noose.
I like Monkeybar’s ADS too, but use it a little differently. I got a knee brace, and connect the ADS to the knee brace with a rubber band. If I want more tension, I add more rubber bands. That gives you similar tension to an extender, but it is vastly more comfortable and you can wear it under your clothes with fewer problems.