Is one type of testament ‘better’ than the other from a genealogical point of view?

Yes. You will find a ‘testament testamentar’ much more informative. As this includes a copy of the will of the deceased, you will be able to discover exactly how your ancestor wished to dispose of his or her possessions, and who was chosen to wind up the estate. This was usually a member of the family. A ‘testament dative’, on the other hand, does not contain any indication of the wishes of the deceased with regard to the disposal of his or her property since it was not drawn up until after the death. But the person appointed by the court as the executor could well have been the next of kin, so this will supply you with at least one genealogical detail.