Directors Cut is an abused marketing term these days, but there is a difference between a Directors Cut and an extended cut. A directors cut is exactly that - his preferred cut and what he wants you to see. The only film Ridley said he regrets cutting to much out of was Kingdom of Heaven. The release version was not his director's cut, so he put the long DC out on DVD. The original Blade Runner, and the first DVD marketed as his directors cut, were not his director's cuts. The American version of Legend was not his directors cut. The original Alien (according to what he claims now) was not his directors cut. In the intro to one of the longer cut releases, I think it's Gladiator, he says right on the disk that it is not his preferred version.
The release version of Prometheus was always to be his DC. The only question was, would there be a longer cut later, and he said there would not be one before the film even hit theaters.