Being Bi-lingual....have used both techiques....and have had numerous intellectual agruments over a pint or two dozen on the subject. We even did a comparison....a dinosaur from Oz and myself in one of Big Al's lovely Bell 47G2's at the Redhill smashplatz. It would seem the power demand is the same....assuming not reaching ETL prior to crossing the barriers....the arguing point seemed to center over the issue of giving up ground as you ascend using the US Army (FAA) method or being well up in the air should you lose a Donk while using the UK/European method. For twins it is a no brainer.....ascend to a height clear of the barriers then go.....but for singles...it really is a toss up.
GLS....please remember there are two kinds of helicopters.....Chinooks and slingloads. Have never had a problem taking off with a Huey....except when the main blade comes untied and the airframe wants to spin like a top......but then they always did make for an unstable slingload !