Wooden Floor Underlay

Just trying to work out what type of underlay I should be using for a hardwood wooden floor (T&G) I will be installing as a floating floor in my basement. Before I continue, I must point out that the basement is completely sealed with a studded floor membrane that is sealed to the wall membrane. On top of the floor membrane is T&G flooring grade chipboard (green stuff) glued together (floating). The flooring place (Flooring2Go) is trying to get me to use their most expensive Latex rubber underlay which works out at 3.50 p/m2 - this is because the standard white 2mm underlay (which you see in sheds etc) is 'not suitable'. The white stuff I have seen for 1 p/m2 is a considerable saving for the amount I need. I`m dubious at their advice, as of course they want me to spend the max possible. I can`t see why the 2mm underlay would not be suitable especially with the floor underneath. What do people think?
Steve
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