I have both ADA diffusers and cheap Chinese diffusers. The ADAs definitely have higher quality glass, but it's really the ceramic diffuser disc that counts, and all of mine work equally well in that respect. The ADAs just do it in style. The only other thing I can think of is that my ADAs are very consistent from one to the next in terms of the diameter of the glass neck where you attach your tubing to, while the Chinese ones are all over the map. I've had some that the tubing slips off, and others that it seems impossible to get the tubing on to.
It is like buying a Toyota vs a Lexus. Both can get you to work just as well, but one just does it with more style.
Also, just in general don't forget that if you do use CO2 resistant tubing, you shouldn't be actually using that tubing to directly connect to your diffuser. You should be using silicone tubing or something much less stiff. That might help with the cases of individuals not being able to get their tubing on (although bacod253, I'm not sure what your specific case is so what I'm saying may not apply to you!). If you're worried about CO2 leaking through the tubes, then just connect the diffuser with silicone to a check valve, and keep a minimum of the silicone exposed to the air and the rest submerged underwater. Then, from the check valve to your CO2 cylinder you can use the CO2 resistant tubing.