Any carpet fitters on here, need your expert advice

In the process of choosing underlay for the upstairs of my home, 4 bedrooms, landing and staircase. I think I will be going to Blue Heaven (from the makers of cloud 9). It's like Cloud 9 but without the paper backing as this can sometimes be noisy underfoot.
My quandary is, do I go for 9mm or 11mm? I would like 11mm as this offers greater sound impact reduction and thermal insulation. I have read on other forums that some carpet fitters don't like the 11mm as it is harder to stretch the carpets.
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I can't see why.
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*All generalizations are false.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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avoid tearing out.
On the other group, less trafficed than here but perhaps better mannered, I generally direct question posters here for the broader knowledge base. Useful to subscribe to both.
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fred
BBC3, ITV2/3/4, channels going to the DOGs
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