I prefer the Photoflex arm. You can even use it as a light duty hairlight/boom arm. I find the Flexfill arms(with the rubber holders)difficult to use. The ones that I've used always twisted in even the slightest breeze due to not being able to tighten down the lower rubber holder. I've never used the Westcott arm. Bogen/Lastolite also makes an arm that looks identical to the Westcott.
Jtown is right 100% about the flexfill reflector holder, I strongly do not recomend it to anyone. I bought one a couple A total ripoff for ($95.00) and could never figure out how to use it. select another brand.
I have a Bogen and a Photoflex arm. The bogen is absolutely useless in a breeze, the clips on it are far too weak to hold the reflector.
The Photoflex is a ripoff of the Matthews (MSE) version. The handle that tightens it up actually has MSE stamped on it. This arm is heaps better. It has a bunch of different adjustments, you are able to attach a counter weight to it if need be and it will certainly hold a Pro light. The biggest advantage that I find is that the clips are very firm. Much more stable in a breeze.