Stacking is still the best, walang masyadong pressure sa case and the discs. Kung piling, the case at the bottom of the pile might rub/squeeze on the disc. One of the problems of HD DVD discs are they are rather very fragile, and sensitive. A minor swath with a finger is enough to leave a mark. Unlike BD discs, which are impregnable to scratches, unless you really have the intention of really destroying it. Put the BD collection on the bottom kung ganon.