Apart from the price what the differance and why would you go for top one??
Have onlyt heard good reviews about this membrane:
The RO UltraTec HF5-4040 has no equivilant product at the moment from any of the other membrane manufacturers ranges. This membrane will replace and out perform any of the membranes lised below
Thats was taken from the website.
If it can make water quicker at lower pressure has to be a good thing, most things that save time in my book is worth paying extra for.
I will be replacing my 4040's with these membranes when they are due to be renewed