Without a doubt, vitrification. This technique is the one that has really made it possible to offer the technique of female preservation with all the guarantees.
First of all it would be necessary to distinguish the two techniques and their history, as well as why the conventional freezing technique was relatively valid for embryos but not for oocytes.
With the conventional freezing technique, cryoprotectants are used in low concentration for a relatively long time and a slow progressive cooling. This sometimes results in the formation of crystals that are lethal to the cells. The larger the cell, the greater the possibility of crystals forming, and if we take into account that the egg is the largest cell in the human body, this explains why this technique has produced very poor results with eggs.
The vitrification technique consists of exposing embryos and oocytes to high concentrations of cryoprotectant in a relatively short time and ultra-fast cooling (in the order of 20,000 ºC/min). As a result of this process a vitreous mass is formed (hence the name vitrification) avoiding the formation of crystals that would damage the cell.
Therefore, vitrification of eggs is undoubtedly better than freezing. It is this technique that has really made it possible to offer the technique of female preservation in a standardized way with all the guarantees.