Ok ! Take some pritives in Bryce and give them a volumetric material. This produces the most realistic cloud-look in my opinion. The hypervoxel look is good as well but it has still something artifical. It is too tight... more like thick smoke. I have at least not seen any cloud material render from lightwave that would be more realistic than the bryce volumetric cloud look. (However Bryce is ultra slow and has problems with volumetric materials used together with grouped objects-.....but...nevermind...)