While I don't see even half a dozen, they're all based on the same Asmedia chip, AFAICT (Renesas makes some, too, but nobody seems to use them for these enclosures--I can't find any, at least). Speed will be about the same with all of them, and will be most affected by SSD installed, host controller used, and driver used. The ones that get super fast file benchmark scores all use SF drives inside, and are taking advantage of compressible data.
TBH, one of the biggest problems with them right now will be that since we're transitioning away from mSATA, nobody wants to make new ones, and are instead waiting on M.2 to proliferate, even though that's going to take awhile yet.
I can't say anything personal on specific ones, though. I will say I am surprised that most external USB 3.0 SSDs are standard 2.5" drives. I was thinking of suggesting to get one, and swap out the SSD, but they're almost all 3" wide or more, on top of their expense compared to an M500 or Evo and a an enclosure.