Of the three I have had experience with, I prefer Gerber. The American Standard Flushmate-equipped Cadet I used to have was noisy and tended to leave tissue and other matter in the bowl unless such matter was squarely over or very near the hole in the bottom of the bowl. I have noticed that the design has changed recently so it may no longer be an issue.
I found Kohler's Cadet with Flushmate to have insufficient bowl wash. The flush lever also was, to me, awkward and a little difficult to operate.
I don't know for certain that the Gerber is any better than the other two, but I have not noticed any of the problems with it that I mentioned above. I have had no experience with any other Flushmate-equipped brands.